Merge branch 'master' into custom-discarded-job-handling

119 files changed, 1562 insertions, 2094 deletions

diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index 0c19fed9e1..5de6503144 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -27,14 +27,14 @@ require "active_support"
 require "active_support/rails"
 require "active_model"
 require "arel"
+require "yaml"
 
-require_relative "active_record/version"
-require_relative "active_record/attribute_set"
+require "active_record/version"
+require "active_model/attribute_set"
 
 module ActiveRecord
   extend ActiveSupport::Autoload
 
-  autoload :Attribute
   autoload :Base
   autoload :Callbacks
   autoload :Core
@@ -104,6 +104,7 @@ module ActiveRecord
 
     autoload :Result
     autoload :TableMetadata
+    autoload :Type
   end
 
   module Coders
@@ -181,3 +182,7 @@ end
 ActiveSupport.on_load(:i18n) do
   I18n.load_path << File.expand_path("active_record/locale/en.yml", __dir__)
 end
+
+YAML.load_tags["!ruby/object:ActiveRecord::AttributeSet"] = "ActiveModel::AttributeSet"
+YAML.load_tags["!ruby/object:ActiveRecord::Attribute::FromDatabase"] = "ActiveModel::Attribute::FromDatabase"
+YAML.load_tags["!ruby/object:ActiveRecord::LazyAttributeHash"] = "ActiveModel::LazyAttributeHash"
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index a61c0336db..661605d3e5 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -3,7 +3,7 @@
 require "active_support/core_ext/enumerable"
 require "active_support/core_ext/string/conversions"
 require "active_support/core_ext/module/remove_method"
-require_relative "errors"
+require "active_record/errors"
 
 module ActiveRecord
   class AssociationNotFoundError < ConfigurationError #:nodoc:
@@ -140,26 +140,6 @@ module ActiveRecord
   class HasOneThroughCantAssociateThroughHasOneOrManyReflection < ThroughCantAssociateThroughHasOneOrManyReflection #:nodoc:
   end
 
-  class HasManyThroughCantAssociateNewRecords < ActiveRecordError #:nodoc:
-    def initialize(owner = nil, reflection = nil)
-      if owner && reflection
-        super("Cannot associate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to create the has_many :through record associating them.")
-      else
-        super("Cannot associate new records.")
-      end
-    end
-  end
-
-  class HasManyThroughCantDissociateNewRecords < ActiveRecordError #:nodoc:
-    def initialize(owner = nil, reflection = nil)
-      if owner && reflection
-        super("Cannot dissociate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to delete the has_many :through record associating them.")
-      else
-        super("Cannot dissociate new records.")
-      end
-    end
-  end
-
   class ThroughNestedAssociationsAreReadonly < ActiveRecordError #:nodoc:
     def initialize(owner = nil, reflection = nil)
       if owner && reflection
@@ -189,16 +169,6 @@ module ActiveRecord
     end
   end
 
-  class ReadOnlyAssociation < ActiveRecordError #:nodoc:
-    def initialize(reflection = nil)
-      if reflection
-        super("Cannot add to a has_many :through association. Try adding to #{reflection.through_reflection.name.inspect}.")
-      else
-        super("Read-only reflection error.")
-      end
-    end
-  end
-
   # This error is raised when trying to destroy a parent instance in N:1 or 1:1 associations
   # (has_many, has_one) when there is at least 1 child associated instance.
   # ex: if @project.tasks.size > 0, DeleteRestrictionError will be raised when trying to destroy @project
@@ -483,14 +453,14 @@ module ActiveRecord
       # The tables for these classes could look something like:
       #
       #   CREATE TABLE users (
-      #     id int NOT NULL auto_increment,
-      #     account_id int default NULL,
+      #     id bigint NOT NULL auto_increment,
+      #     account_id bigint default NULL,
       #     name varchar default NULL,
       #     PRIMARY KEY  (id)
       #   )
       #
       #   CREATE TABLE accounts (
-      #     id int NOT NULL auto_increment,
+      #     id bigint NOT NULL auto_increment,
       #     name varchar default NULL,
       #     PRIMARY KEY  (id)
       #   )
@@ -557,9 +527,8 @@ module ActiveRecord
       #     has_many :birthday_events, ->(user) { where(starts_on: user.birthday) }, class_name: 'Event'
       #   end
       #
-      # Note: Joining, eager loading and preloading of these associations is not fully possible.
+      # Note: Joining, eager loading and preloading of these associations is not possible.
       # These operations happen before instance creation and the scope will be called with a +nil+ argument.
-      # This can lead to unexpected behavior and is deprecated.
       #
       # == Association callbacks
       #
@@ -850,7 +819,7 @@ module ActiveRecord
       #   project.milestones             # fetches milestones from the database
       #   project.milestones.size        # uses the milestone cache
       #   project.milestones.empty?      # uses the milestone cache
-      #   project.milestones(true).size  # fetches milestones from the database
+      #   project.milestones.reload.size # fetches milestones from the database
       #   project.milestones             # uses the milestone cache
       #
       # == Eager loading of associations
@@ -1400,7 +1369,7 @@ module ActiveRecord
         #   has_many :tags, as: :taggable
         #   has_many :reports, -> { readonly }
         #   has_many :subscribers, through: :subscriptions, source: :user
-        def has_many(name, scope = nil, options = {}, &extension)
+        def has_many(name, scope = nil, **options, &extension)
           reflection = Builder::HasMany.build(self, name, scope, options, &extension)
           Reflection.add_reflection self, name, reflection
         end
@@ -1534,7 +1503,7 @@ module ActiveRecord
         #   has_one :club, through: :membership
         #   has_one :primary_address, -> { where(primary: true) }, through: :addressables, source: :addressable
         #   has_one :credit_card, required: true
-        def has_one(name, scope = nil, options = {})
+        def has_one(name, scope = nil, **options)
           reflection = Builder::HasOne.build(self, name, scope, options)
           Reflection.add_reflection self, name, reflection
         end
@@ -1678,7 +1647,7 @@ module ActiveRecord
         #   belongs_to :company, touch: :employees_last_updated_at
         #   belongs_to :user, optional: true
         #   belongs_to :account, default: -> { company.account }
-        def belongs_to(name, scope = nil, options = {})
+        def belongs_to(name, scope = nil, **options)
           reflection = Builder::BelongsTo.build(self, name, scope, options)
           Reflection.add_reflection self, name, reflection
         end
@@ -1848,7 +1817,7 @@ module ActiveRecord
 
           builder = Builder::HasAndBelongsToMany.new name, self, options
 
-          join_model = ActiveSupport::Deprecation.silence { builder.through_model }
+          join_model = builder.through_model
 
           const_set join_model.name, join_model
           private_constant join_model.name
@@ -1877,7 +1846,7 @@ module ActiveRecord
             hm_options[k] = options[k] if options.key? k
           end
 
-          ActiveSupport::Deprecation.silence { has_many name, scope, hm_options, &extension }
+          has_many name, scope, hm_options, &extension
           _reflections[name.to_s].parent_reflection = habtm_reflection
         end
       end
diff --git a/activerecord/lib/active_record/associations/alias_tracker.rb b/activerecord/lib/active_record/associations/alias_tracker.rb
index 096f016976..14881cfe17 100644
--- a/activerecord/lib/active_record/associations/alias_tracker.rb
+++ b/activerecord/lib/active_record/associations/alias_tracker.rb
@@ -6,36 +6,34 @@ module ActiveRecord
   module Associations
     # Keeps track of table aliases for ActiveRecord::Associations::JoinDependency
     class AliasTracker # :nodoc:
-      def self.create(connection, initial_table)
-        aliases = Hash.new(0)
-        aliases[initial_table] = 1
-        new(connection, aliases)
-      end
-
-      def self.create_with_joins(connection, initial_table, joins)
+      def self.create(connection, initial_table, joins)
         if joins.empty?
-          create(connection, initial_table)
+          aliases = Hash.new(0)
         else
           aliases = Hash.new { |h, k|
             h[k] = initial_count_for(connection, k, joins)
           }
-          aliases[initial_table] = 1
-          new(connection, aliases)
         end
+        aliases[initial_table] = 1
+        new(connection, aliases)
       end
 
       def self.initial_count_for(connection, name, table_joins)
-        # quoted_name should be downcased as some database adapters (Oracle) return quoted name in uppercase
-        quoted_name = connection.quote_table_name(name).downcase
+        quoted_name = nil
 
         counts = table_joins.map do |join|
           if join.is_a?(Arel::Nodes::StringJoin)
+            # quoted_name should be case ignored as some database adapters (Oracle) return quoted name in uppercase
+            quoted_name ||= connection.quote_table_name(name)
+
             # Table names + table aliases
-            join.left.downcase.scan(
-              /join(?:\s+\w+)?\s+(\S+\s+)?#{quoted_name}\son/
+            join.left.scan(
+              /JOIN(?:\s+\w+)?\s+(?:\S+\s+)?(?:#{quoted_name}|#{name})\sON/i
             ).size
           elsif join.respond_to? :left
-            join.left.table_name == name ? 1 : 0
+            join.left.name == name ? 1 : 0
+          elsif join.is_a?(Hash)
+            join.fetch(name, 0)
           else
             # this branch is reached by two tests:
             #
@@ -79,10 +77,7 @@ module ActiveRecord
         end
       end
 
-      # TODO Change this to private once we've dropped Ruby 2.2 support.
-      # Workaround for Ruby 2.2 "private attribute?" warning.
-      protected
-        attr_reader :aliases
+      attr_reader :aliases
 
       private
 
diff --git a/activerecord/lib/active_record/associations/association_scope.rb b/activerecord/lib/active_record/associations/association_scope.rb
index b2dc044312..11967e0571 100644
--- a/activerecord/lib/active_record/associations/association_scope.rb
+++ b/activerecord/lib/active_record/associations/association_scope.rb
@@ -23,8 +23,7 @@ module ActiveRecord
         reflection = association.reflection
         scope = klass.unscoped
         owner = association.owner
-        alias_tracker = AliasTracker.create(klass.connection, klass.table_name)
-        chain = get_chain(reflection, association, alias_tracker)
+        chain = get_chain(reflection, association, scope.alias_tracker)
 
         scope.extending! reflection.extensions
         add_constraints(scope, owner, chain)
diff --git a/activerecord/lib/active_record/associations/builder/association.rb b/activerecord/lib/active_record/associations/builder/association.rb
index 496b16b58f..ca3032d967 100644
--- a/activerecord/lib/active_record/associations/builder/association.rb
+++ b/activerecord/lib/active_record/associations/builder/association.rb
@@ -38,11 +38,6 @@ module ActiveRecord::Associations::Builder # :nodoc:
     def self.create_reflection(model, name, scope, options, extension = nil)
       raise ArgumentError, "association names must be a Symbol" unless name.kind_of?(Symbol)
 
-      if scope.is_a?(Hash)
-        options = scope
-        scope   = nil
-      end
-
       validate_options(options)
 
       scope = build_scope(scope, extension)
diff --git a/activerecord/lib/active_record/associations/builder/collection_association.rb b/activerecord/lib/active_record/associations/builder/collection_association.rb
index 753fde5146..35a72c3850 100644
--- a/activerecord/lib/active_record/associations/builder/collection_association.rb
+++ b/activerecord/lib/active_record/associations/builder/collection_association.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "../../associations"
+require "active_record/associations"
 
 module ActiveRecord::Associations::Builder # :nodoc:
   class CollectionAssociation < Association #:nodoc:
diff --git a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
index 12fcfbcd45..1981da11a2 100644
--- a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
+++ b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
@@ -47,7 +47,7 @@ module ActiveRecord::Associations::Builder # :nodoc:
       habtm = JoinTableResolver.build lhs_model, association_name, options
 
       join_model = Class.new(ActiveRecord::Base) {
-        class << self;
+        class << self
           attr_accessor :left_model
           attr_accessor :name
           attr_accessor :table_name_resolver
diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index ceedf150e3..921237a735 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -79,7 +79,13 @@ module ActiveRecord
       def find(*args)
         if options[:inverse_of] && loaded?
           args_flatten = args.flatten
-          raise RecordNotFound, "Couldn't find #{scope.klass.name} without an ID" if args_flatten.blank?
+          model = scope.klass
+
+          if args_flatten.blank?
+            error_message = "Couldn't find #{model.name} without an ID"
+            raise RecordNotFound.new(error_message, model.name, model.primary_key, args)
+          end
+
           result = find_by_scan(*args)
 
           result_size = Array(result).size
@@ -181,8 +187,6 @@ module ActiveRecord
       # are actually removed from the database, that depends precisely on
       # +delete_records+. They are in any case removed from the collection.
       def delete(*records)
-        return if records.empty?
-        records = find(records) if records.any? { |record| record.kind_of?(Integer) || record.kind_of?(String) }
         delete_or_destroy(records, options[:dependent])
       end
 
@@ -192,8 +196,6 @@ module ActiveRecord
       # Note that this method removes records from the database ignoring the
       # +:dependent+ option.
       def destroy(*records)
-        return if records.empty?
-        records = find(records) if records.any? { |record| record.kind_of?(Integer) || record.kind_of?(String) }
         delete_or_destroy(records, :destroy)
       end
 
@@ -376,6 +378,8 @@ module ActiveRecord
         end
 
         def delete_or_destroy(records, method)
+          return if records.empty?
+          records = find(records) if records.any? { |record| record.kind_of?(Integer) || record.kind_of?(String) }
           records = records.flatten
           records.each { |record| raise_on_type_mismatch!(record) }
           existing_records = records.reject(&:new_record?)
diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index 412e89255d..8b4a48a38c 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -988,6 +988,12 @@ module ActiveRecord
         load_target == other
       end
 
+      ##
+      # :method: to_ary
+      #
+      # :call-seq:
+      #   to_ary()
+      #
       # Returns a new array of objects from the collection. If the collection
       # hasn't been loaded, it fetches the records from the database.
       #
@@ -1021,10 +1027,6 @@ module ActiveRecord
       #   #       #<Pet id: 5, name: "Brain", person_id: 1>,
       #   #       #<Pet id: 6, name: "Boss",  person_id: 1>
       #   #    ]
-      def to_ary
-        load_target.dup
-      end
-      alias_method :to_a, :to_ary
 
       def records # :nodoc:
         load_target
@@ -1072,7 +1074,6 @@ module ActiveRecord
       end
 
       # Reloads the collection from the database. Returns +self+.
-      # Equivalent to <tt>collection(true)</tt>.
       #
       #   class Person < ActiveRecord::Base
       #     has_many :pets
@@ -1086,9 +1087,6 @@ module ActiveRecord
       #
       #   person.pets.reload # fetches pets from the database
       #   # => [#<Pet id: 1, name: "Snoop", group: "dogs", person_id: 1>]
-      #
-      #   person.pets(true)  # fetches pets from the database
-      #   # => [#<Pet id: 1, name: "Snoop", group: "dogs", person_id: 1>]
       def reload
         proxy_association.reload
         reset_scope
diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index 77b3d11b47..df4bf07999 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -43,8 +43,6 @@ module ActiveRecord
         Column = Struct.new(:name, :alias)
       end
 
-      attr_reader :alias_tracker, :base_klass, :join_root
-
       def self.make_tree(associations)
         hash = {}
         walk_tree associations, hash
@@ -90,8 +88,8 @@ module ActiveRecord
       #    associations # => [:appointments]
       #    joins # =>  []
       #
-      def initialize(base, table, associations, joins, eager_loading: true)
-        @alias_tracker = AliasTracker.create_with_joins(base.connection, base.table_name, joins)
+      def initialize(base, table, associations, alias_tracker, eager_loading: true)
+        @alias_tracker = alias_tracker
         @eager_loading = eager_loading
         tree = self.class.make_tree associations
         @join_root = JoinBase.new(base, table, build(tree, base))
@@ -158,6 +156,9 @@ module ActiveRecord
         parents.values
       end
 
+      protected
+        attr_reader :alias_tracker, :base_klass, :join_root
+
       private
 
         def make_constraints(parent, child, tables, join_type)
@@ -224,7 +225,7 @@ module ActiveRecord
               raise EagerLoadPolymorphicError.new(reflection)
             end
 
-            JoinAssociation.new reflection, build(right, reflection.klass)
+            JoinAssociation.new(reflection, build(right, reflection.klass), alias_tracker)
           end.compact
         end
 
diff --git a/activerecord/lib/active_record/associations/join_dependency/join_association.rb b/activerecord/lib/active_record/associations/join_dependency/join_association.rb
index a526468bf6..221c791bf8 100644
--- a/activerecord/lib/active_record/associations/join_dependency/join_association.rb
+++ b/activerecord/lib/active_record/associations/join_dependency/join_association.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "join_part"
+require "active_record/associations/join_dependency/join_part"
 
 module ActiveRecord
   module Associations
@@ -11,11 +11,12 @@ module ActiveRecord
 
         attr_accessor :tables
 
-        def initialize(reflection, children)
+        def initialize(reflection, children, alias_tracker)
           super(reflection.klass, children)
 
-          @reflection      = reflection
-          @tables          = nil
+          @alias_tracker = alias_tracker
+          @reflection    = reflection
+          @tables        = nil
         end
 
         def match?(other)
@@ -38,11 +39,12 @@ module ActiveRecord
             joins << table.create_join(table, table.create_on(constraint), join_type)
 
             join_scope = reflection.join_scope(table, foreign_klass)
+            arel = join_scope.arel(alias_tracker.aliases)
 
-            if join_scope.arel.constraints.any?
-              joins.concat join_scope.arel.join_sources
+            if arel.constraints.any?
+              joins.concat arel.join_sources
               right = joins.last.right
-              right.expr = right.expr.and(join_scope.arel.constraints)
+              right.expr = right.expr.and(arel.constraints)
             end
 
             # The current table in this iteration becomes the foreign table in the next
@@ -55,6 +57,9 @@ module ActiveRecord
         def table
           tables.first
         end
+
+        protected
+          attr_reader :alias_tracker
       end
     end
   end
diff --git a/activerecord/lib/active_record/associations/join_dependency/join_base.rb b/activerecord/lib/active_record/associations/join_dependency/join_base.rb
index 8a8fa8993b..988b4e8fa2 100644
--- a/activerecord/lib/active_record/associations/join_dependency/join_base.rb
+++ b/activerecord/lib/active_record/associations/join_dependency/join_base.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "join_part"
+require "active_record/associations/join_dependency/join_part"
 
 module ActiveRecord
   module Associations
diff --git a/activerecord/lib/active_record/associations/preloader.rb b/activerecord/lib/active_record/associations/preloader.rb
index 62caf02a2c..e1087be9b3 100644
--- a/activerecord/lib/active_record/associations/preloader.rb
+++ b/activerecord/lib/active_record/associations/preloader.rb
@@ -44,16 +44,8 @@ module ActiveRecord
       extend ActiveSupport::Autoload
 
       eager_autoload do
-        autoload :Association,           "active_record/associations/preloader/association"
-        autoload :SingularAssociation,   "active_record/associations/preloader/singular_association"
-        autoload :CollectionAssociation, "active_record/associations/preloader/collection_association"
-        autoload :ThroughAssociation,    "active_record/associations/preloader/through_association"
-
-        autoload :HasMany,             "active_record/associations/preloader/has_many"
-        autoload :HasManyThrough,      "active_record/associations/preloader/has_many_through"
-        autoload :HasOne,              "active_record/associations/preloader/has_one"
-        autoload :HasOneThrough,       "active_record/associations/preloader/has_one_through"
-        autoload :BelongsTo,           "active_record/associations/preloader/belongs_to"
+        autoload :Association,        "active_record/associations/preloader/association"
+        autoload :ThroughAssociation, "active_record/associations/preloader/through_association"
       end
 
       # Eager loads the named associations for the given Active Record record(s).
@@ -91,13 +83,13 @@ module ActiveRecord
       #   { author: :avatar }
       #   [ :books, { author: :avatar } ]
       def preload(records, associations, preload_scope = nil)
-        records       = Array.wrap(records).compact.uniq
-        associations  = Array.wrap(associations)
+        records = records.compact
 
         if records.empty?
           []
         else
-          associations.flat_map { |association|
+          records.uniq!
+          Array.wrap(associations).flat_map { |association|
             preloaders_on association, records, preload_scope
           }
         end
@@ -166,8 +158,6 @@ module ActiveRecord
         end
 
         class AlreadyLoaded # :nodoc:
-          attr_reader :owners, :reflection
-
           def initialize(klass, owners, reflection, preload_scope)
             @owners = owners
             @reflection = reflection
@@ -178,11 +168,13 @@ module ActiveRecord
           def preloaded_records
             owners.flat_map { |owner| owner.association(reflection.name).target }
           end
+
+          protected
+            attr_reader :owners, :reflection
         end
 
         # Returns a class containing the logic needed to load preload the data
-        # and attach it to a relation. For example +Preloader::Association+ or
-        # +Preloader::HasManyThrough+. The class returned implements a `run` method
+        # and attach it to a relation. The class returned implements a `run` method
         # that accepts a preloader.
         def preloader_for(reflection, owners)
           if owners.first.association(reflection.name).loaded?
@@ -190,13 +182,10 @@ module ActiveRecord
           end
           reflection.check_preloadable!
 
-          case reflection.macro
-          when :has_many
-            reflection.options[:through] ? HasManyThrough : HasMany
-          when :has_one
-            reflection.options[:through] ? HasOneThrough : HasOne
-          when :belongs_to
-            BelongsTo
+          if reflection.options[:through]
+            ThroughAssociation
+          else
+            Association
           end
         end
     end
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index 9bb6a613e1..735da152b7 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -4,7 +4,6 @@ module ActiveRecord
   module Associations
     class Preloader
       class Association #:nodoc:
-        attr_reader :owners, :reflection, :preload_scope, :model, :klass
         attr_reader :preloaded_records
 
         def initialize(klass, owners, reflection, preload_scope)
@@ -17,55 +16,50 @@ module ActiveRecord
         end
 
         def run(preloader)
-          preload(preloader)
-        end
-
-        def preload(preloader)
-          raise NotImplementedError
-        end
+          records = load_records do |record|
+            owner = owners_by_key[convert_key(record[association_key_name])]
+            association = owner.association(reflection.name)
+            association.set_inverse_instance(record)
+          end
 
-        # The name of the key on the associated records
-        def association_key_name
-          raise NotImplementedError
+          owners.each do |owner|
+            associate_records_to_owner(owner, records[convert_key(owner[owner_key_name])] || [])
+          end
         end
 
-        # The name of the key on the model which declares the association
-        def owner_key_name
-          raise NotImplementedError
-        end
+        protected
+          attr_reader :owners, :reflection, :preload_scope, :model, :klass
 
         private
-          def options
-            reflection.options
+          # The name of the key on the associated records
+          def association_key_name
+            reflection.join_primary_key(klass)
           end
 
-          def associated_records_by_owner(preloader)
-            records = load_records do |record|
-              owner = owners_by_key[convert_key(record[association_key_name])]
-              association = owner.association(reflection.name)
-              association.set_inverse_instance(record)
-            end
+          # The name of the key on the model which declares the association
+          def owner_key_name
+            reflection.join_foreign_key
+          end
 
-            owners.each_with_object({}) do |owner, result|
-              result[owner] = records[convert_key(owner[owner_key_name])] || []
+          def associate_records_to_owner(owner, records)
+            association = owner.association(reflection.name)
+            if reflection.collection?
+              association.loaded!
+              association.target.concat(records)
+            else
+              association.target = records.first
             end
           end
 
           def owner_keys
-            unless defined?(@owner_keys)
-              @owner_keys = owners.map do |owner|
-                owner[owner_key_name]
-              end
-              @owner_keys.uniq!
-              @owner_keys.compact!
-            end
-            @owner_keys
+            @owner_keys ||= owners_by_key.keys
           end
 
           def owners_by_key
             unless defined?(@owners_by_key)
               @owners_by_key = owners.each_with_object({}) do |owner, h|
-                h[convert_key(owner[owner_key_name])] = owner
+                key = convert_key(owner[owner_key_name])
+                h[key] = owner if key
               end
             end
             @owners_by_key
diff --git a/activerecord/lib/active_record/associations/preloader/belongs_to.rb b/activerecord/lib/active_record/associations/preloader/belongs_to.rb
deleted file mode 100644
index ae9695f26a..0000000000
--- a/activerecord/lib/active_record/associations/preloader/belongs_to.rb
+++ /dev/null
@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class BelongsTo < SingularAssociation #:nodoc:
-        def association_key_name
-          options[:primary_key] || klass && klass.primary_key
-        end
-
-        def owner_key_name
-          reflection.foreign_key
-        end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/collection_association.rb b/activerecord/lib/active_record/associations/preloader/collection_association.rb
deleted file mode 100644
index fb920a642c..0000000000
--- a/activerecord/lib/active_record/associations/preloader/collection_association.rb
+++ /dev/null
@@ -1,19 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class CollectionAssociation < Association #:nodoc:
-        private
-
-          def preload(preloader)
-            associated_records_by_owner(preloader).each do |owner, records|
-              association = owner.association(reflection.name)
-              association.loaded!
-              association.target.concat(records)
-            end
-          end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_many.rb b/activerecord/lib/active_record/associations/preloader/has_many.rb
deleted file mode 100644
index 29a1ce099d..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_many.rb
+++ /dev/null
@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasMany < CollectionAssociation #:nodoc:
-        def association_key_name
-          reflection.foreign_key
-        end
-
-        def owner_key_name
-          reflection.active_record_primary_key
-        end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_many_through.rb b/activerecord/lib/active_record/associations/preloader/has_many_through.rb
deleted file mode 100644
index 0639fdca44..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_many_through.rb
+++ /dev/null
@@ -1,21 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasManyThrough < CollectionAssociation #:nodoc:
-        include ThroughAssociation
-
-        def associated_records_by_owner(preloader)
-          records_by_owner = super
-
-          if reflection_scope.distinct_value
-            records_by_owner.each_value(&:uniq!)
-          end
-
-          records_by_owner
-        end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_one.rb b/activerecord/lib/active_record/associations/preloader/has_one.rb
deleted file mode 100644
index d87abf630f..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_one.rb
+++ /dev/null
@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasOne < SingularAssociation #:nodoc:
-        def association_key_name
-          reflection.foreign_key
-        end
-
-        def owner_key_name
-          reflection.active_record_primary_key
-        end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_one_through.rb b/activerecord/lib/active_record/associations/preloader/has_one_through.rb
deleted file mode 100644
index 17734d0257..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_one_through.rb
+++ /dev/null
@@ -1,11 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasOneThrough < SingularAssociation #:nodoc:
-        include ThroughAssociation
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/singular_association.rb b/activerecord/lib/active_record/associations/preloader/singular_association.rb
deleted file mode 100644
index 266b5f6b1c..0000000000
--- a/activerecord/lib/active_record/associations/preloader/singular_association.rb
+++ /dev/null
@@ -1,20 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class SingularAssociation < Association #:nodoc:
-        private
-
-          def preload(preloader)
-            associated_records_by_owner(preloader).each do |owner, associated_records|
-              record = associated_records.first
-
-              association = owner.association(reflection.name)
-              association.target = record
-            end
-          end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 8aac00d910..a6b7ab80a2 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -3,84 +3,60 @@
 module ActiveRecord
   module Associations
     class Preloader
-      module ThroughAssociation #:nodoc:
-        def through_reflection
-          reflection.through_reflection
-        end
-
-        def source_reflection
-          reflection.source_reflection
-        end
-
-        def associated_records_by_owner(preloader)
-          through_scope = through_scope()
-
-          preloader.preload(owners,
-                            through_reflection.name,
-                            through_scope)
-
-          through_records = owners.map do |owner|
-            center = owner.association(through_reflection.name).target
-            [owner, Array(center)]
-          end
-
-          reset_association(owners, through_reflection.name, through_scope)
-
-          middle_records = through_records.flat_map(&:last)
-
-          preloaders = preloader.preload(middle_records,
-                                         source_reflection.name,
-                                         reflection_scope)
-
+      class ThroughAssociation < Association # :nodoc:
+        def run(preloader)
+          already_loaded     = owners.first.association(through_reflection.name).loaded?
+          through_scope      = through_scope()
+          reflection_scope   = target_reflection_scope
+          through_preloaders = preloader.preload(owners, through_reflection.name, through_scope)
+          middle_records     = through_preloaders.flat_map(&:preloaded_records)
+          preloaders         = preloader.preload(middle_records, source_reflection.name, reflection_scope)
           @preloaded_records = preloaders.flat_map(&:preloaded_records)
 
-          middle_to_pl = preloaders.each_with_object({}) do |pl, h|
-            pl.owners.each { |middle|
-              h[middle] = pl
-            }
-          end
-
-          through_records.each_with_object({}) do |(lhs, center), records_by_owner|
-            pl_to_middle = center.group_by { |record| middle_to_pl[record] }
-
-            records_by_owner[lhs] = pl_to_middle.flat_map do |pl, middles|
-              rhs_records = middles.flat_map { |r|
-                r.association(source_reflection.name).target
-              }.compact
-
-              # Respect the order on `reflection_scope` if it exists, else use the natural order.
-              if reflection_scope.values[:order].present?
-                @id_map ||= id_to_index_map @preloaded_records
-                rhs_records.sort_by { |rhs| @id_map[rhs] }
-              else
-                rhs_records
+          owners.each do |owner|
+            through_records = Array(owner.association(through_reflection.name).target)
+            if already_loaded
+              if source_type = reflection.options[:source_type]
+                through_records = through_records.select do |record|
+                  record[reflection.foreign_type] == source_type
+                end
               end
+            else
+              owner.association(through_reflection.name).reset if through_scope
+            end
+            result = through_records.flat_map do |record|
+              association = record.association(source_reflection.name)
+              target = association.target
+              association.reset if preload_scope
+              target
+            end
+            result.compact!
+            if reflection_scope
+              result.sort_by! { |rhs| preload_index[rhs] } if reflection_scope.order_values.any?
+              result.uniq! if reflection_scope.distinct_value
             end
+            associate_records_to_owner(owner, result)
           end
         end
 
         private
-
-          def id_to_index_map(ids)
-            id_map = {}
-            ids.each_with_index { |id, index| id_map[id] = index }
-            id_map
+          def through_reflection
+            reflection.through_reflection
           end
 
-          def reset_association(owners, association_name, through_scope)
-            should_reset = (through_scope != through_reflection.klass.unscoped) ||
-               (options[:source_type] && through_reflection.collection?)
+          def source_reflection
+            reflection.source_reflection
+          end
 
-            # Don't cache the association - we would only be caching a subset
-            if should_reset
-              owners.each { |owner|
-                owner.association(association_name).reset
-              }
+          def preload_index
+            @preload_index ||= @preloaded_records.each_with_object({}).with_index do |(id, result), index|
+              result[id] = index
             end
           end
 
           def through_scope
             scope = through_reflection.klass.unscoped
+            options = reflection.options
 
             if options[:source_type]
               scope.where! reflection.foreign_type => options[:source_type]
@@ -113,7 +89,17 @@ module ActiveRecord
               end
             end
 
-            scope
+            scope unless scope.empty_scope?
+          end
+
+          def target_reflection_scope
+            if preload_scope
+              reflection_scope.merge(preload_scope)
+            elsif reflection.scope
+              reflection_scope
+            else
+              nil
+            end
           end
       end
     end
diff --git a/activerecord/lib/active_record/attribute.rb b/activerecord/lib/active_record/attribute.rb
deleted file mode 100644
index fc474edc15..0000000000
--- a/activerecord/lib/active_record/attribute.rb
+++ /dev/null
@@ -1,242 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  class Attribute # :nodoc:
-    class << self
-      def from_database(name, value, type)
-        FromDatabase.new(name, value, type)
-      end
-
-      def from_user(name, value, type, original_attribute = nil)
-        FromUser.new(name, value, type, original_attribute)
-      end
-
-      def with_cast_value(name, value, type)
-        WithCastValue.new(name, value, type)
-      end
-
-      def null(name)
-        Null.new(name)
-      end
-
-      def uninitialized(name, type)
-        Uninitialized.new(name, type)
-      end
-    end
-
-    attr_reader :name, :value_before_type_cast, :type
-
-    # This method should not be called directly.
-    # Use #from_database or #from_user
-    def initialize(name, value_before_type_cast, type, original_attribute = nil)
-      @name = name
-      @value_before_type_cast = value_before_type_cast
-      @type = type
-      @original_attribute = original_attribute
-    end
-
-    def value
-      # `defined?` is cheaper than `||=` when we get back falsy values
-      @value = type_cast(value_before_type_cast) unless defined?(@value)
-      @value
-    end
-
-    def original_value
-      if assigned?
-        original_attribute.original_value
-      else
-        type_cast(value_before_type_cast)
-      end
-    end
-
-    def value_for_database
-      type.serialize(value)
-    end
-
-    def changed?
-      changed_from_assignment? || changed_in_place?
-    end
-
-    def changed_in_place?
-      has_been_read? && type.changed_in_place?(original_value_for_database, value)
-    end
-
-    def forgetting_assignment
-      with_value_from_database(value_for_database)
-    end
-
-    def with_value_from_user(value)
-      type.assert_valid_value(value)
-      self.class.from_user(name, value, type, original_attribute || self)
-    end
-
-    def with_value_from_database(value)
-      self.class.from_database(name, value, type)
-    end
-
-    def with_cast_value(value)
-      self.class.with_cast_value(name, value, type)
-    end
-
-    def with_type(type)
-      if changed_in_place?
-        with_value_from_user(value).with_type(type)
-      else
-        self.class.new(name, value_before_type_cast, type, original_attribute)
-      end
-    end
-
-    def type_cast(*)
-      raise NotImplementedError
-    end
-
-    def initialized?
-      true
-    end
-
-    def came_from_user?
-      false
-    end
-
-    def has_been_read?
-      defined?(@value)
-    end
-
-    def ==(other)
-      self.class == other.class &&
-        name == other.name &&
-        value_before_type_cast == other.value_before_type_cast &&
-        type == other.type
-    end
-    alias eql? ==
-
-    def hash
-      [self.class, name, value_before_type_cast, type].hash
-    end
-
-    def init_with(coder)
-      @name = coder["name"]
-      @value_before_type_cast = coder["value_before_type_cast"]
-      @type = coder["type"]
-      @original_attribute = coder["original_attribute"]
-      @value = coder["value"] if coder.map.key?("value")
-    end
-
-    def encode_with(coder)
-      coder["name"] = name
-      coder["value_before_type_cast"] = value_before_type_cast unless value_before_type_cast.nil?
-      coder["type"] = type if type
-      coder["original_attribute"] = original_attribute if original_attribute
-      coder["value"] = value if defined?(@value)
-    end
-
-    # TODO Change this to private once we've dropped Ruby 2.2 support.
-    # Workaround for Ruby 2.2 "private attribute?" warning.
-    protected
-
-      attr_reader :original_attribute
-      alias_method :assigned?, :original_attribute
-
-      def original_value_for_database
-        if assigned?
-          original_attribute.original_value_for_database
-        else
-          _original_value_for_database
-        end
-      end
-
-    private
-      def initialize_dup(other)
-        if defined?(@value) && @value.duplicable?
-          @value = @value.dup
-        end
-      end
-
-      def changed_from_assignment?
-        assigned? && type.changed?(original_value, value, value_before_type_cast)
-      end
-
-      def _original_value_for_database
-        type.serialize(original_value)
-      end
-
-      class FromDatabase < Attribute # :nodoc:
-        def type_cast(value)
-          type.deserialize(value)
-        end
-
-        def _original_value_for_database
-          value_before_type_cast
-        end
-      end
-
-      class FromUser < Attribute # :nodoc:
-        def type_cast(value)
-          type.cast(value)
-        end
-
-        def came_from_user?
-          !type.value_constructed_by_mass_assignment?(value_before_type_cast)
-        end
-      end
-
-      class WithCastValue < Attribute # :nodoc:
-        def type_cast(value)
-          value
-        end
-
-        def changed_in_place?
-          false
-        end
-      end
-
-      class Null < Attribute # :nodoc:
-        def initialize(name)
-          super(name, nil, Type.default_value)
-        end
-
-        def type_cast(*)
-          nil
-        end
-
-        def with_type(type)
-          self.class.with_cast_value(name, nil, type)
-        end
-
-        def with_value_from_database(value)
-          raise ActiveModel::MissingAttributeError, "can't write unknown attribute `#{name}`"
-        end
-        alias_method :with_value_from_user, :with_value_from_database
-      end
-
-      class Uninitialized < Attribute # :nodoc:
-        UNINITIALIZED_ORIGINAL_VALUE = Object.new
-
-        def initialize(name, type)
-          super(name, nil, type)
-        end
-
-        def value
-          if block_given?
-            yield name
-          end
-        end
-
-        def original_value
-          UNINITIALIZED_ORIGINAL_VALUE
-        end
-
-        def value_for_database
-        end
-
-        def initialized?
-          false
-        end
-
-        def with_type(type)
-          self.class.new(name, type)
-        end
-      end
-      private_constant :FromDatabase, :FromUser, :Null, :Uninitialized, :WithCastValue
-  end
-end
diff --git a/activerecord/lib/active_record/attribute/user_provided_default.rb b/activerecord/lib/active_record/attribute/user_provided_default.rb
deleted file mode 100644
index 690a931615..0000000000
--- a/activerecord/lib/active_record/attribute/user_provided_default.rb
+++ /dev/null
@@ -1,32 +0,0 @@
-# frozen_string_literal: true
-
-require_relative "../attribute"
-
-module ActiveRecord
-  class Attribute # :nodoc:
-    class UserProvidedDefault < FromUser # :nodoc:
-      def initialize(name, value, type, database_default)
-        @user_provided_value = value
-        super(name, value, type, database_default)
-      end
-
-      def value_before_type_cast
-        if user_provided_value.is_a?(Proc)
-          @memoized_value_before_type_cast ||= user_provided_value.call
-        else
-          @user_provided_value
-        end
-      end
-
-      def with_type(type)
-        self.class.new(name, user_provided_value, type, original_attribute)
-      end
-
-      # TODO Change this to private once we've dropped Ruby 2.2 support.
-      # Workaround for Ruby 2.2 "private attribute?" warning.
-      protected
-
-        attr_reader :user_provided_value
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index e4ca6c8408..64f81ca582 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -33,7 +33,9 @@ module ActiveRecord
 
     BLACKLISTED_CLASS_METHODS = %w(private public protected allocate new name parent superclass)
 
-    class GeneratedAttributeMethods < Module; end # :nodoc:
+    class GeneratedAttributeMethods < Module #:nodoc:
+      include Mutex_m
+    end
 
     module ClassMethods
       def inherited(child_class) #:nodoc:
@@ -42,7 +44,7 @@ module ActiveRecord
       end
 
       def initialize_generated_modules # :nodoc:
-        @generated_attribute_methods = GeneratedAttributeMethods.new { extend Mutex_m }
+        @generated_attribute_methods = GeneratedAttributeMethods.new
         @attribute_methods_generated = false
         include @generated_attribute_methods
 
@@ -165,6 +167,46 @@ module ActiveRecord
         end
       end
 
+      # Regexp whitelist. Matches the following:
+      #   "#{table_name}.#{column_name}"
+      #   "#{column_name}"
+      COLUMN_NAME_WHITELIST = /\A(?:\w+\.)?\w+\z/i
+
+      # Regexp whitelist. Matches the following:
+      #   "#{table_name}.#{column_name}"
+      #   "#{table_name}.#{column_name} #{direction}"
+      #   "#{column_name}"
+      #   "#{column_name} #{direction}"
+      COLUMN_NAME_ORDER_WHITELIST = /\A(?:\w+\.)?\w+(?:\s+asc|\s+desc)?\z/i
+
+      def enforce_raw_sql_whitelist(args, whitelist: COLUMN_NAME_WHITELIST) # :nodoc:
+        unexpected = args.reject do |arg|
+          arg.kind_of?(Arel::Node) ||
+            arg.is_a?(Arel::Nodes::SqlLiteral) ||
+            arg.is_a?(Arel::Attributes::Attribute) ||
+            arg.to_s.split(/\s*,\s*/).all? { |part| whitelist.match?(part) }
+        end
+
+        return if unexpected.none?
+
+        if allow_unsafe_raw_sql == :deprecated
+          ActiveSupport::Deprecation.warn(
+            "Dangerous query method (method whose arguments are used as raw " \
+            "SQL) called with non-attribute argument(s): " \
+            "#{unexpected.map(&:inspect).join(", ")}. Non-attribute " \
+            "arguments will be disallowed in Rails 6.0. This method should " \
+            "not be called with user-provided values, such as request " \
+            "parameters or model attributes. Known-safe values can be passed " \
+            "by wrapping them in Arel.sql()."
+          )
+        else
+          raise(ActiveRecord::UnknownAttributeReference,
+            "Query method called with non-attribute argument(s): " +
+            unexpected.map(&:inspect).join(", ")
+          )
+        end
+      end
+
       # Returns true if the given attribute exists, otherwise false.
       #
       #   class Person < ActiveRecord::Base
@@ -234,7 +276,7 @@ module ActiveRecord
         return has_attribute?(name)
       end
 
-      return true
+      true
     end
 
     # Returns +true+ if the given attribute is in the attributes hash, otherwise +false+.
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 4f957ac3ca..3de6fe566d 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/module/attribute_accessors"
-require_relative "../attribute_mutation_tracker"
 
 module ActiveRecord
   module AttributeMethods
@@ -33,66 +32,13 @@ module ActiveRecord
       # <tt>reload</tt> the record and clears changed attributes.
       def reload(*)
         super.tap do
+          @previously_changed = ActiveSupport::HashWithIndifferentAccess.new
           @mutations_before_last_save = nil
-          clear_mutation_trackers
-          @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
-        end
-      end
-
-      def initialize_dup(other) # :nodoc:
-        super
-        @attributes = self.class._default_attributes.map do |attr|
-          attr.with_value_from_user(@attributes.fetch_value(attr.name))
-        end
-        clear_mutation_trackers
-      end
-
-      def changes_applied # :nodoc:
-        @mutations_before_last_save = mutation_tracker
-        @mutations_from_database = AttributeMutationTracker.new(@attributes)
-        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
-        forget_attribute_assignments
-        clear_mutation_trackers
-      end
-
-      def clear_changes_information # :nodoc:
-        @mutations_before_last_save = nil
-        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
-        forget_attribute_assignments
-        clear_mutation_trackers
-      end
-
-      def clear_attribute_changes(attr_names) # :nodoc:
-        super
-        attr_names.each do |attr_name|
-          clear_attribute_change(attr_name)
-        end
-      end
-
-      def changed_attributes # :nodoc:
-        # This should only be set by methods which will call changed_attributes
-        # multiple times when it is known that the computed value cannot change.
-        if defined?(@cached_changed_attributes)
-          @cached_changed_attributes
-        else
-          super.reverse_merge(mutation_tracker.changed_values).freeze
-        end
-      end
-
-      def changes # :nodoc:
-        cache_changed_attributes do
-          super
+          @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
+          @mutations_from_database = nil
         end
       end
 
-      def previous_changes # :nodoc:
-        mutations_before_last_save.changes
-      end
-
-      def attribute_changed_in_place?(attr_name) # :nodoc:
-        mutation_tracker.changed_in_place?(attr_name)
-      end
-
       # Did this attribute change when we last saved? This method can be invoked
       # as +saved_change_to_name?+ instead of <tt>saved_change_to_attribute?("name")</tt>.
       # Behaves similarly to +attribute_changed?+. This method is useful in
@@ -183,34 +129,6 @@ module ActiveRecord
           result
         end
 
-        def mutation_tracker
-          unless defined?(@mutation_tracker)
-            @mutation_tracker = nil
-          end
-          @mutation_tracker ||= AttributeMutationTracker.new(@attributes)
-        end
-
-        def mutations_from_database
-          unless defined?(@mutations_from_database)
-            @mutations_from_database = nil
-          end
-          @mutations_from_database ||= mutation_tracker
-        end
-
-        def changes_include?(attr_name)
-          super || mutation_tracker.changed?(attr_name)
-        end
-
-        def clear_attribute_change(attr_name)
-          mutation_tracker.forget_change(attr_name)
-          mutations_from_database.forget_change(attr_name)
-        end
-
-        def attribute_will_change!(attr_name)
-          super
-          mutations_from_database.force_change(attr_name)
-        end
-
         def _update_record(*)
           partial_writes? ? super(keys_for_partial_write) : super
         end
@@ -222,30 +140,6 @@ module ActiveRecord
         def keys_for_partial_write
           changed_attribute_names_to_save & self.class.column_names
         end
-
-        def forget_attribute_assignments
-          @attributes = @attributes.map(&:forgetting_assignment)
-        end
-
-        def clear_mutation_trackers
-          @mutation_tracker = nil
-          @mutations_from_database = nil
-        end
-
-        def mutations_before_last_save
-          @mutations_before_last_save ||= NullMutationTracker.instance
-        end
-
-        def cache_changed_attributes
-          @cached_changed_attributes = changed_attributes
-          yield
-        ensure
-          clear_changed_attributes_cache
-        end
-
-        def clear_changed_attributes_cache
-          remove_instance_variable(:@cached_changed_attributes) if defined?(@cached_changed_attributes)
-        end
     end
   end
 end
diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 63c059e291..d8fc046e10 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -17,13 +17,15 @@ module ActiveRecord
       # Returns the primary key value.
       def id
         sync_with_transaction_state
-        _read_attribute(self.class.primary_key) if self.class.primary_key
+        primary_key = self.class.primary_key
+        _read_attribute(primary_key) if primary_key
       end
 
       # Sets the primary key value.
       def id=(value)
         sync_with_transaction_state
-        _write_attribute(self.class.primary_key, value) if self.class.primary_key
+        primary_key = self.class.primary_key
+        _write_attribute(primary_key, value) if primary_key
       end
 
       # Queries the primary key value.
diff --git a/activerecord/lib/active_record/attribute_methods/read.rb b/activerecord/lib/active_record/attribute_methods/read.rb
index b070235684..4077250583 100644
--- a/activerecord/lib/active_record/attribute_methods/read.rb
+++ b/activerecord/lib/active_record/attribute_methods/read.rb
@@ -58,8 +58,9 @@ module ActiveRecord
           attr_name.to_s
         end
 
-        name = self.class.primary_key if name == "id".freeze && self.class.primary_key
-        sync_with_transaction_state if name == self.class.primary_key
+        primary_key = self.class.primary_key
+        name = primary_key if name == "id".freeze && primary_key
+        sync_with_transaction_state if name == primary_key
         _read_attribute(name, &block)
       end
 
diff --git a/activerecord/lib/active_record/attribute_methods/write.rb b/activerecord/lib/active_record/attribute_methods/write.rb
index 37891ce2ef..bb0ec6a8c3 100644
--- a/activerecord/lib/active_record/attribute_methods/write.rb
+++ b/activerecord/lib/active_record/attribute_methods/write.rb
@@ -39,8 +39,9 @@ module ActiveRecord
           attr_name.to_s
         end
 
-        name = self.class.primary_key if name == "id".freeze && self.class.primary_key
-        sync_with_transaction_state if name == self.class.primary_key
+        primary_key = self.class.primary_key
+        name = primary_key if name == "id".freeze && primary_key
+        sync_with_transaction_state if name == primary_key
         _write_attribute(name, value)
       end
 
diff --git a/activerecord/lib/active_record/attribute_mutation_tracker.rb b/activerecord/lib/active_record/attribute_mutation_tracker.rb
deleted file mode 100644
index 94bf641a5d..0000000000
--- a/activerecord/lib/active_record/attribute_mutation_tracker.rb
+++ /dev/null
@@ -1,111 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  class AttributeMutationTracker # :nodoc:
-    OPTION_NOT_GIVEN = Object.new
-
-    def initialize(attributes)
-      @attributes = attributes
-      @forced_changes = Set.new
-    end
-
-    def changed_values
-      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
-        if changed?(attr_name)
-          result[attr_name] = attributes[attr_name].original_value
-        end
-      end
-    end
-
-    def changes
-      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
-        change = change_to_attribute(attr_name)
-        if change
-          result[attr_name] = change
-        end
-      end
-    end
-
-    def change_to_attribute(attr_name)
-      attr_name = attr_name.to_s
-      if changed?(attr_name)
-        [attributes[attr_name].original_value, attributes.fetch_value(attr_name)]
-      end
-    end
-
-    def any_changes?
-      attr_names.any? { |attr| changed?(attr) }
-    end
-
-    def changed?(attr_name, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN)
-      attr_name = attr_name.to_s
-      forced_changes.include?(attr_name) ||
-        attributes[attr_name].changed? &&
-        (OPTION_NOT_GIVEN == from || attributes[attr_name].original_value == from) &&
-        (OPTION_NOT_GIVEN == to || attributes[attr_name].value == to)
-    end
-
-    def changed_in_place?(attr_name)
-      attributes[attr_name.to_s].changed_in_place?
-    end
-
-    def forget_change(attr_name)
-      attr_name = attr_name.to_s
-      attributes[attr_name] = attributes[attr_name].forgetting_assignment
-      forced_changes.delete(attr_name)
-    end
-
-    def original_value(attr_name)
-      attributes[attr_name.to_s].original_value
-    end
-
-    def force_change(attr_name)
-      forced_changes << attr_name.to_s
-    end
-
-    # TODO Change this to private once we've dropped Ruby 2.2 support.
-    # Workaround for Ruby 2.2 "private attribute?" warning.
-    protected
-
-      attr_reader :attributes, :forced_changes
-
-    private
-
-      def attr_names
-        attributes.keys
-      end
-  end
-
-  class NullMutationTracker # :nodoc:
-    include Singleton
-
-    def changed_values(*)
-      {}
-    end
-
-    def changes(*)
-      {}
-    end
-
-    def change_to_attribute(attr_name)
-    end
-
-    def any_changes?(*)
-      false
-    end
-
-    def changed?(*)
-      false
-    end
-
-    def changed_in_place?(*)
-      false
-    end
-
-    def forget_change(*)
-    end
-
-    def original_value(*)
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/attribute_set.rb b/activerecord/lib/active_record/attribute_set.rb
deleted file mode 100644
index 492067e2b3..0000000000
--- a/activerecord/lib/active_record/attribute_set.rb
+++ /dev/null
@@ -1,113 +0,0 @@
-# frozen_string_literal: true
-
-require_relative "attribute_set/builder"
-require_relative "attribute_set/yaml_encoder"
-
-module ActiveRecord
-  class AttributeSet # :nodoc:
-    delegate :each_value, :fetch, to: :attributes
-
-    def initialize(attributes)
-      @attributes = attributes
-    end
-
-    def [](name)
-      attributes[name] || Attribute.null(name)
-    end
-
-    def []=(name, value)
-      attributes[name] = value
-    end
-
-    def values_before_type_cast
-      attributes.transform_values(&:value_before_type_cast)
-    end
-
-    def to_hash
-      initialized_attributes.transform_values(&:value)
-    end
-    alias_method :to_h, :to_hash
-
-    def key?(name)
-      attributes.key?(name) && self[name].initialized?
-    end
-
-    def keys
-      attributes.each_key.select { |name| self[name].initialized? }
-    end
-
-    if defined?(JRUBY_VERSION)
-      # This form is significantly faster on JRuby, and this is one of our biggest hotspots.
-      # https://github.com/jruby/jruby/pull/2562
-      def fetch_value(name, &block)
-        self[name].value(&block)
-      end
-    else
-      def fetch_value(name)
-        self[name].value { |n| yield n if block_given? }
-      end
-    end
-
-    def write_from_database(name, value)
-      attributes[name] = self[name].with_value_from_database(value)
-    end
-
-    def write_from_user(name, value)
-      attributes[name] = self[name].with_value_from_user(value)
-    end
-
-    def write_cast_value(name, value)
-      attributes[name] = self[name].with_cast_value(value)
-    end
-
-    def freeze
-      @attributes.freeze
-      super
-    end
-
-    def deep_dup
-      self.class.allocate.tap do |copy|
-        copy.instance_variable_set(:@attributes, attributes.deep_dup)
-      end
-    end
-
-    def initialize_dup(_)
-      @attributes = attributes.dup
-      super
-    end
-
-    def initialize_clone(_)
-      @attributes = attributes.clone
-      super
-    end
-
-    def reset(key)
-      if key?(key)
-        write_from_database(key, nil)
-      end
-    end
-
-    def accessed
-      attributes.select { |_, attr| attr.has_been_read? }.keys
-    end
-
-    def map(&block)
-      new_attributes = attributes.transform_values(&block)
-      AttributeSet.new(new_attributes)
-    end
-
-    def ==(other)
-      attributes == other.attributes
-    end
-
-    protected
-
-      attr_reader :attributes
-
-    private
-
-      def initialized_attributes
-        attributes.select { |_, attr| attr.initialized? }
-      end
-  end
-end
diff --git a/activerecord/lib/active_record/attribute_set/builder.rb b/activerecord/lib/active_record/attribute_set/builder.rb
deleted file mode 100644
index e3a9c7fdb3..0000000000
--- a/activerecord/lib/active_record/attribute_set/builder.rb
+++ /dev/null
@@ -1,126 +0,0 @@
-# frozen_string_literal: true
-
-require_relative "../attribute"
-
-module ActiveRecord
-  class AttributeSet # :nodoc:
-    class Builder # :nodoc:
-      attr_reader :types, :always_initialized, :default
-
-      def initialize(types, always_initialized = nil, &default)
-        @types = types
-        @always_initialized = always_initialized
-        @default = default
-      end
-
-      def build_from_database(values = {}, additional_types = {})
-        if always_initialized && !values.key?(always_initialized)
-          values[always_initialized] = nil
-        end
-
-        attributes = LazyAttributeHash.new(types, values, additional_types, &default)
-        AttributeSet.new(attributes)
-      end
-    end
-  end
-
-  class LazyAttributeHash # :nodoc:
-    delegate :transform_values, :each_key, :each_value, :fetch, to: :materialize
-
-    def initialize(types, values, additional_types, &default)
-      @types = types
-      @values = values
-      @additional_types = additional_types
-      @materialized = false
-      @delegate_hash = {}
-      @default = default || proc {}
-    end
-
-    def key?(key)
-      delegate_hash.key?(key) || values.key?(key) || types.key?(key)
-    end
-
-    def [](key)
-      delegate_hash[key] || assign_default_value(key)
-    end
-
-    def []=(key, value)
-      if frozen?
-        raise RuntimeError, "Can't modify frozen hash"
-      end
-      delegate_hash[key] = value
-    end
-
-    def deep_dup
-      dup.tap do |copy|
-        copy.instance_variable_set(:@delegate_hash, delegate_hash.transform_values(&:dup))
-      end
-    end
-
-    def initialize_dup(_)
-      @delegate_hash = Hash[delegate_hash]
-      super
-    end
-
-    def select
-      keys = types.keys | values.keys | delegate_hash.keys
-      keys.each_with_object({}) do |key, hash|
-        attribute = self[key]
-        if yield(key, attribute)
-          hash[key] = attribute
-        end
-      end
-    end
-
-    def ==(other)
-      if other.is_a?(LazyAttributeHash)
-        materialize == other.materialize
-      else
-        materialize == other
-      end
-    end
-
-    def marshal_dump
-      materialize
-    end
-
-    def marshal_load(delegate_hash)
-      @delegate_hash = delegate_hash
-      @types = {}
-      @values = {}
-      @additional_types = {}
-      @materialized = true
-    end
-
-    # TODO Change this to private once we've dropped Ruby 2.2 support.
-    # Workaround for Ruby 2.2 "private attribute?" warning.
-    protected
-
-      attr_reader :types, :values, :additional_types, :delegate_hash, :default
-
-      def materialize
-        unless @materialized
-          values.each_key { |key| self[key] }
-          types.each_key { |key| self[key] }
-          unless frozen?
-            @materialized = true
-          end
-        end
-        delegate_hash
-      end
-
-    private
-
-      def assign_default_value(name)
-        type = additional_types.fetch(name, types[name])
-        value_present = true
-        value = values.fetch(name) { value_present = false }
-
-        if value_present
-          delegate_hash[name] = Attribute.from_database(name, value, type)
-        elsif types.key?(name)
-          delegate_hash[name] = default.call(name) || Attribute.uninitialized(name, type)
-        end
-      end
-  end
-end
diff --git a/activerecord/lib/active_record/attribute_set/yaml_encoder.rb b/activerecord/lib/active_record/attribute_set/yaml_encoder.rb
deleted file mode 100644
index 9254ce16ab..0000000000
--- a/activerecord/lib/active_record/attribute_set/yaml_encoder.rb
+++ /dev/null
@@ -1,43 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  class AttributeSet
-    # Attempts to do more intelligent YAML dumping of an
-    # ActiveRecord::AttributeSet to reduce the size of the resulting string
-    class YAMLEncoder # :nodoc:
-      def initialize(default_types)
-        @default_types = default_types
-      end
-
-      def encode(attribute_set, coder)
-        coder["concise_attributes"] = attribute_set.each_value.map do |attr|
-          if attr.type.equal?(default_types[attr.name])
-            attr.with_type(nil)
-          else
-            attr
-          end
-        end
-      end
-
-      def decode(coder)
-        if coder["attributes"]
-          coder["attributes"]
-        else
-          attributes_hash = Hash[coder["concise_attributes"].map do |attr|
-            if attr.type.nil?
-              attr = attr.with_type(default_types[attr.name])
-            end
-            [attr.name, attr]
-          end]
-          AttributeSet.new(attributes_hash)
-        end
-      end
-
-      # TODO Change this to private once we've dropped Ruby 2.2 support.
-      # Workaround for Ruby 2.2 "private attribute?" warning.
-      protected
-
-        attr_reader :default_types
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/attributes.rb b/activerecord/lib/active_record/attributes.rb
index afb559db71..0b7c9398a8 100644
--- a/activerecord/lib/active_record/attributes.rb
+++ b/activerecord/lib/active_record/attributes.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "attribute/user_provided_default"
+require "active_model/attribute/user_provided_default"
 
 module ActiveRecord
   # See ActiveRecord::Attributes::ClassMethods for documentation
@@ -250,14 +250,14 @@ module ActiveRecord
           if value == NO_DEFAULT_PROVIDED
             default_attribute = _default_attributes[name].with_type(type)
           elsif from_user
-            default_attribute = Attribute::UserProvidedDefault.new(
+            default_attribute = ActiveModel::Attribute::UserProvidedDefault.new(
               name,
               value,
               type,
               _default_attributes.fetch(name.to_s) { nil },
             )
           else
-            default_attribute = Attribute.from_database(name, value, type)
+            default_attribute = ActiveModel::Attribute.from_database(name, value, type)
           end
           _default_attributes[name] = default_attribute
         end
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index 6974cf74f6..a1250c3835 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -436,6 +436,9 @@ module ActiveRecord
             if (autosave && record.changed_for_autosave?) || new_record? || record_changed?(reflection, record, key)
               unless reflection.through_reflection
                 record[reflection.foreign_key] = key
+                if inverse_reflection = reflection.inverse_of
+                  record.association(inverse_reflection.name).loaded!
+                end
               end
 
               saved = record.save(validate: !autosave)
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index 541ff51fbe..b7ad944cec 100644
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -15,14 +15,14 @@ require "active_support/core_ext/kernel/singleton_class"
 require "active_support/core_ext/module/introspection"
 require "active_support/core_ext/object/duplicable"
 require "active_support/core_ext/class/subclasses"
-require_relative "attribute_decorators"
-require_relative "define_callbacks"
-require_relative "errors"
-require_relative "log_subscriber"
-require_relative "explain_subscriber"
-require_relative "relation/delegation"
-require_relative "attributes"
-require_relative "type_caster"
+require "active_record/attribute_decorators"
+require "active_record/define_callbacks"
+require "active_record/errors"
+require "active_record/log_subscriber"
+require "active_record/explain_subscriber"
+require "active_record/relation/delegation"
+require "active_record/attributes"
+require "active_record/type_caster"
 
 module ActiveRecord #:nodoc:
   # = Active Record
diff --git a/activerecord/lib/active_record/callbacks.rb b/activerecord/lib/active_record/callbacks.rb
index a2439e6ec7..9ab2780760 100644
--- a/activerecord/lib/active_record/callbacks.rb
+++ b/activerecord/lib/active_record/callbacks.rb
@@ -98,9 +98,9 @@ module ActiveRecord
   # == Types of callbacks
   #
   # There are four types of callbacks accepted by the callback macros: Method references (symbol), callback objects,
-  # inline methods (using a proc), and inline eval methods (using a string). Method references and callback objects
+  # inline methods (using a proc). Method references and callback objects
   # are the recommended approaches, inline methods using a proc are sometimes appropriate (such as for
-  # creating mix-ins), and inline eval methods are deprecated.
+  # creating mix-ins).
   #
   # The method reference callbacks work by specifying a protected or private method available in the object, like this:
   #
@@ -240,7 +240,7 @@ module ActiveRecord
   #
   #     private
   #
-  #     def log_chidren
+  #     def log_children
   #       # Child processing
   #     end
   #
@@ -265,7 +265,7 @@ module ActiveRecord
   #
   #     private
   #
-  #     def log_chidren
+  #     def log_children
   #       # Child processing
   #     end
   #
diff --git a/activerecord/lib/active_record/collection_cache_key.rb b/activerecord/lib/active_record/collection_cache_key.rb
index f3e6516414..88b398ad45 100644
--- a/activerecord/lib/active_record/collection_cache_key.rb
+++ b/activerecord/lib/active_record/collection_cache_key.rb
@@ -12,6 +12,9 @@ module ActiveRecord
           timestamp = collection.max_by(&timestamp_column)._read_attribute(timestamp_column)
         end
       else
+        if collection.eager_loading?
+          collection = collection.send(:apply_join_dependency)
+        end
         column_type = type_for_attribute(timestamp_column.to_s)
         column = connection.column_name_from_arel_node(collection.arel_attribute(timestamp_column))
         select_values = "COUNT(*) AS #{connection.quote_column_name("size")}, MAX(%s) AS timestamp"
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index 0759f4d2b3..9849f9d5d7 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -63,15 +63,13 @@ module ActiveRecord
     # There are several connection-pooling-related options that you can add to
     # your database connection configuration:
     #
-    # * +pool+: number indicating size of connection pool (default 5)
-    # * +checkout_timeout+: number of seconds to block and wait for a connection
-    #   before giving up and raising a timeout error (default 5 seconds).
-    # * +reaping_frequency+: frequency in seconds to periodically run the
-    #   Reaper, which attempts to find and recover connections from dead
-    #   threads, which can occur if a programmer forgets to close a
-    #   connection at the end of a thread or a thread dies unexpectedly.
-    #   Regardless of this setting, the Reaper will be invoked before every
-    #   blocking wait. (Default +nil+, which means don't schedule the Reaper).
+    # * +pool+: maximum number of connections the pool may manage (default 5).
+    # * +idle_timeout+: number of seconds that a connection will be kept
+    #   unused in the pool before it is automatically disconnected (default
+    #   300 seconds). Set this to zero to keep connections forever.
+    # * +checkout_timeout+: number of seconds to wait for a connection to
+    #   become available before giving up and raising a timeout error (default
+    #   5 seconds).
     #
     #--
     # Synchronization policy:
@@ -82,11 +80,8 @@ module ActiveRecord
     # * private methods that require being called in a +synchronize+ blocks
     #   are now explicitly documented
     class ConnectionPool
-      # Threadsafe, fair, FIFO queue.  Meant to be used by ConnectionPool
-      # with which it shares a Monitor.  But could be a generic Queue.
-      #
-      # The Queue in stdlib's 'thread' could replace this class except
-      # stdlib's doesn't support waiting with a timeout.
+      # Threadsafe, fair, LIFO queue.  Meant to be used by ConnectionPool
+      # with which it shares a Monitor.
       class Queue
         def initialize(lock = Monitor.new)
           @lock = lock
@@ -175,7 +170,7 @@ module ActiveRecord
 
           # Removes and returns the head of the queue if possible, or +nil+.
           def remove
-            @queue.shift
+            @queue.pop
           end
 
           # Remove and return the head the queue if the number of
@@ -283,12 +278,12 @@ module ActiveRecord
           end
       end
 
-      # Every +frequency+ seconds, the reaper will call +reap+ on +pool+.
-      # A reaper instantiated with a +nil+ frequency will never reap the
-      # connection pool.
+      # Every +frequency+ seconds, the reaper will call +reap+ and +flush+ on
+      # +pool+. A reaper instantiated with a zero frequency will never reap
+      # the connection pool.
       #
-      # Configure the frequency by setting "reaping_frequency" in your
-      # database yaml file.
+      # Configure the frequency by setting +reaping_frequency+ in your database
+      # yaml file (default 60 seconds).
       class Reaper
         attr_reader :pool, :frequency
 
@@ -298,11 +293,12 @@ module ActiveRecord
         end
 
         def run
-          return unless frequency
+          return unless frequency && frequency > 0
           Thread.new(frequency, pool) { |t, p|
             loop do
               sleep t
               p.reap
+              p.flush
             end
           }
         end
@@ -326,6 +322,10 @@ module ActiveRecord
         @spec = spec
 
         @checkout_timeout = (spec.config[:checkout_timeout] && spec.config[:checkout_timeout].to_f) || 5
+        if @idle_timeout = spec.config.fetch(:idle_timeout, 300)
+          @idle_timeout = @idle_timeout.to_f
+          @idle_timeout = nil if @idle_timeout <= 0
+        end
 
         # default max pool size to 5
         @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5
@@ -356,7 +356,10 @@ module ActiveRecord
 
         @lock_thread = false
 
-        @reaper = Reaper.new(self, spec.config[:reaping_frequency] && spec.config[:reaping_frequency].to_f)
+        # +reaping_frequency+ is configurable mostly for historical reasons, but it could
+        # also be useful if someone wants a very low +idle_timeout+.
+        reaping_frequency = spec.config.fetch(:reaping_frequency, 60)
+        @reaper = Reaper.new(self, reaping_frequency && reaping_frequency.to_f)
         @reaper.run
       end
 
@@ -450,6 +453,21 @@ module ActiveRecord
         disconnect(false)
       end
 
+      # Discards all connections in the pool (even if they're currently
+      # leased!), along with the pool itself. Any further interaction with the
+      # pool (except #spec and #schema_cache) is undefined.
+      #
+      # See AbstractAdapter#discard!
+      def discard! # :nodoc:
+        synchronize do
+          return if @connections.nil? # already discarded
+          @connections.each do |conn|
+            conn.discard!
+          end
+          @connections = @available = @thread_cached_conns = nil
+        end
+      end
+
       # Clears the cache which maps classes and re-connects connections that
       # require reloading.
       #
@@ -575,6 +593,35 @@ module ActiveRecord
         end
       end
 
+      # Disconnect all connections that have been idle for at least
+      # +minimum_idle+ seconds. Connections currently checked out, or that were
+      # checked in less than +minimum_idle+ seconds ago, are unaffected.
+      def flush(minimum_idle = @idle_timeout)
+        return if minimum_idle.nil?
+
+        idle_connections = synchronize do
+          @connections.select do |conn|
+            !conn.in_use? && conn.seconds_idle >= minimum_idle
+          end.each do |conn|
+            conn.lease
+
+            @available.delete conn
+            @connections.delete conn
+          end
+        end
+
+        idle_connections.each do |conn|
+          conn.disconnect!
+        end
+      end
+
+      # Disconnect all currently idle connections. Connections currently checked
+      # out are unaffected.
+      def flush!
+        reap
+        flush(-1)
+      end
+
       def num_waiting_in_queue # :nodoc:
         @available.num_waiting
       end
@@ -866,11 +913,31 @@ module ActiveRecord
     # about the model. The model needs to pass a specification name to the handler,
     # in order to look up the correct connection pool.
     class ConnectionHandler
+      def self.unowned_pool_finalizer(pid_map) # :nodoc:
+        lambda do |_|
+          discard_unowned_pools(pid_map)
+        end
+      end
+
+      def self.discard_unowned_pools(pid_map) # :nodoc:
+        pid_map.each do |pid, pools|
+          pools.values.compact.each(&:discard!) unless pid == Process.pid
+        end
+      end
+
       def initialize
         # These caches are keyed by spec.name (ConnectionSpecification#name).
         @owner_to_pool = Concurrent::Map.new(initial_capacity: 2) do |h, k|
+          # Discard the parent's connection pools immediately; we have no need
+          # of them
+          ConnectionHandler.discard_unowned_pools(h)
+
           h[k] = Concurrent::Map.new(initial_capacity: 2)
         end
+
+        # Backup finalizer: if the forked child never needed a pool, the above
+        # early discard has not occurred
+        ObjectSpace.define_finalizer self, ConnectionHandler.unowned_pool_finalizer(@owner_to_pool)
       end
 
       def connection_pool_list
@@ -924,6 +991,13 @@ module ActiveRecord
         connection_pool_list.each(&:disconnect!)
       end
 
+      # Disconnects all currently idle connections.
+      #
+      # See ConnectionPool#flush! for details.
+      def flush_idle_connections!
+        connection_pool_list.each(&:flush!)
+      end
+
       # Locate the connection of the nearest super class. This can be an
       # active or defined connection: if it is the latter, it will be
       # opened and set as the active connection for the class it was defined
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb b/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb
index 9ad04c3216..92e46ccf9f 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb
@@ -11,19 +11,6 @@ module ActiveRecord
       def quote(value)
         value = id_value_for_database(value) if value.is_a?(Base)
 
-        if value.respond_to?(:quoted_id)
-          at = value.method(:quoted_id).source_location
-          at &&= " at %s:%d" % at
-
-          owner = value.method(:quoted_id).owner.to_s
-          klass = value.class.to_s
-          klass += "(#{owner})" unless owner == klass
-
-          ActiveSupport::Deprecation.warn \
-            "Defining #quoted_id is deprecated and will be ignored in Rails 5.2. (defined on #{klass}#{at})"
-          return value.quoted_id
-        end
-
         if value.respond_to?(:value_for_database)
           value = value.value_for_database
         end
@@ -37,10 +24,6 @@ module ActiveRecord
       def type_cast(value, column = nil)
         value = id_value_for_database(value) if value.is_a?(Base)
 
-        if value.respond_to?(:quoted_id) && value.respond_to?(:id)
-          return value.id
-        end
-
         if column
           value = type_cast_from_column(column, value)
         end
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index 788a455773..0594b4b485 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -6,7 +6,7 @@ module ActiveRecord
     # this type are typically created and returned by methods in database
     # adapters. e.g. ActiveRecord::ConnectionAdapters::MySQL::SchemaStatements#indexes
     class IndexDefinition # :nodoc:
-      attr_reader :table, :name, :unique, :columns, :lengths, :orders, :where, :type, :using, :comment
+      attr_reader :table, :name, :unique, :columns, :lengths, :orders, :opclasses, :where, :type, :using, :comment
 
       def initialize(
         table, name,
@@ -14,6 +14,7 @@ module ActiveRecord
         columns = [],
         lengths: {},
         orders: {},
+        opclasses: {},
         where: nil,
         type: nil,
         using: nil,
@@ -23,13 +24,23 @@ module ActiveRecord
         @name = name
         @unique = unique
         @columns = columns
-        @lengths = lengths
-        @orders = orders
+        @lengths = concise_options(lengths)
+        @orders = concise_options(orders)
+        @opclasses = concise_options(opclasses)
         @where = where
         @type = type
         @using = using
         @comment = comment
       end
+
+      private
+        def concise_options(options)
+          if columns.size == options.size && options.values.uniq.size == 1
+            options.values.first
+          else
+            options
+          end
+        end
     end
 
     # Abstract representation of a column definition. Instances of this type
@@ -85,6 +96,11 @@ module ActiveRecord
         options[:primary_key] != default_primary_key
       end
 
+      def validate?
+        options.fetch(:validate, true)
+      end
+      alias validated? validate?
+
       def defined_for?(to_table_ord = nil, to_table: nil, **options)
         if to_table_ord
           self.to_table == to_table_ord.to_s
@@ -204,6 +220,7 @@ module ActiveRecord
         :decimal,
         :float,
         :integer,
+        :json,
         :string,
         :text,
         :time,
@@ -396,6 +413,9 @@ module ActiveRecord
       alias :belongs_to :references
 
       def new_column_definition(name, type, **options) # :nodoc:
+        if integer_like_primary_key?(type, options)
+          type = integer_like_primary_key_type(type, options)
+        end
         type = aliased_types(type.to_s, type)
         options[:primary_key] ||= type == :primary_key
         options[:null] = false if options[:primary_key]
@@ -410,6 +430,14 @@ module ActiveRecord
         def aliased_types(name, fallback)
           "timestamp" == name ? :datetime : fallback
         end
+
+        def integer_like_primary_key?(type, options)
+          options[:primary_key] && [:integer, :bigint].include?(type) && !options.key?(:default)
+        end
+
+        def integer_like_primary_key_type(type, options)
+          type
+        end
     end
 
     class AlterTable # :nodoc:
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index f57c7a5d4d..4f58b0242c 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
-require_relative "../../migration/join_table"
+require "active_record/migration/join_table"
 require "active_support/core_ext/string/access"
-require "digest"
+require "digest/sha2"
 
 module ActiveRecord
   module ConnectionAdapters # :nodoc:
@@ -79,7 +79,7 @@ module ActiveRecord
       end
 
       # Returns an array of indexes for the given table.
-      def indexes(table_name, name = nil)
+      def indexes(table_name)
         raise NotImplementedError, "#indexes is not implemented"
       end
 
@@ -216,7 +216,7 @@ module ActiveRecord
       # generates:
       #
       #   CREATE TABLE suppliers (
-      #     id int auto_increment PRIMARY KEY
+      #     id bigint auto_increment PRIMARY KEY
       #   ) ENGINE=InnoDB DEFAULT CHARSET=utf8
       #
       # ====== Rename the primary key column
@@ -228,7 +228,7 @@ module ActiveRecord
       # generates:
       #
       #   CREATE TABLE objects (
-      #     guid int auto_increment PRIMARY KEY,
+      #     guid bigint auto_increment PRIMARY KEY,
       #     name varchar(80)
       #   )
       #
@@ -255,8 +255,8 @@ module ActiveRecord
       # generates:
       #
       #   CREATE TABLE order (
-      #       product_id integer NOT NULL,
-      #       client_id integer NOT NULL
+      #       product_id bigint NOT NULL,
+      #       client_id bigint NOT NULL
       #   );
       #
       #   ALTER TABLE ONLY "orders"
@@ -265,15 +265,15 @@ module ActiveRecord
       # ====== Do not add a primary key column
       #
       #   create_table(:categories_suppliers, id: false) do |t|
-      #     t.column :category_id, :integer
-      #     t.column :supplier_id, :integer
+      #     t.column :category_id, :bigint
+      #     t.column :supplier_id, :bigint
       #   end
       #
       # generates:
       #
       #   CREATE TABLE categories_suppliers (
-      #     category_id int,
-      #     supplier_id int
+      #     category_id bigint,
+      #     supplier_id bigint
       #   )
       #
       # ====== Create a temporary table based on a query
@@ -361,8 +361,8 @@ module ActiveRecord
       # generates:
       #
       #   CREATE TABLE assemblies_parts (
-      #     assembly_id int NOT NULL,
-      #     part_id int NOT NULL,
+      #     assembly_id bigint NOT NULL,
+      #     part_id bigint NOT NULL,
       #   ) ENGINE=InnoDB DEFAULT CHARSET=utf8
       #
       def create_join_table(table_1, table_2, column_options: {}, **options)
@@ -432,7 +432,7 @@ module ActiveRecord
       #     t.references :company
       #   end
       #
-      # Creates a <tt>company_id(integer)</tt> column.
+      # Creates a <tt>company_id(bigint)</tt> column.
       #
       # ====== Add a polymorphic foreign key column
       #
@@ -440,7 +440,7 @@ module ActiveRecord
       #    t.belongs_to :company, polymorphic: true
       #  end
       #
-      # Creates <tt>company_type(varchar)</tt> and <tt>company_id(integer)</tt> columns.
+      # Creates <tt>company_type(varchar)</tt> and <tt>company_id(bigint)</tt> columns.
       #
       # ====== Remove a column
       #
@@ -522,6 +522,8 @@ module ActiveRecord
       #   Specifies the precision for the <tt>:decimal</tt> and <tt>:numeric</tt> columns.
       # * <tt>:scale</tt> -
       #   Specifies the scale for the <tt>:decimal</tt> and <tt>:numeric</tt> columns.
+      # * <tt>:comment</tt> -
+      #   Specifies the comment for the column. This option is ignored by some backends.
       #
       # Note: The precision is the total number of significant digits,
       # and the scale is the number of digits that can be stored following
@@ -598,7 +600,7 @@ module ActiveRecord
       # to provide these in a migration's +change+ method so it can be reverted.
       # In that case, +type+ and +options+ will be used by #add_column.
       def remove_column(table_name, column_name, type = nil, options = {})
-        execute "ALTER TABLE #{quote_table_name(table_name)} DROP #{quote_column_name(column_name)}"
+        execute "ALTER TABLE #{quote_table_name(table_name)} #{remove_column_for_alter(table_name, column_name, type, options)}"
       end
 
       # Changes the column's definition according to the new options.
@@ -736,6 +738,28 @@ module ActiveRecord
       #
       # Note: only supported by PostgreSQL and MySQL
       #
+      # ====== Creating an index with a specific operator class
+      #
+      #   add_index(:developers, :name, using: 'gist', opclass: :gist_trgm_ops)
+      #
+      # generates:
+      #
+      #   CREATE INDEX developers_on_name ON developers USING gist (name gist_trgm_ops) -- PostgreSQL
+      #
+      #   add_index(:developers, [:name, :city], using: 'gist', opclass: { city: :gist_trgm_ops })
+      #
+      # generates:
+      #
+      #   CREATE INDEX developers_on_name_and_city ON developers USING gist (name, city gist_trgm_ops) -- PostgreSQL
+      #
+      #   add_index(:developers, [:name, :city], using: 'gist', opclass: :gist_trgm_ops)
+      #
+      # generates:
+      #
+      #   CREATE INDEX developers_on_name_and_city ON developers USING gist (name gist_trgm_ops, city gist_trgm_ops) -- PostgreSQL
+      #
+      # Note: only supported by PostgreSQL
+      #
       # ====== Creating an index with a specific type
       #
       #   add_index(:developers, :name, type: :fulltext)
@@ -804,24 +828,19 @@ module ActiveRecord
       end
 
       # Verifies the existence of an index with a given name.
-      def index_name_exists?(table_name, index_name, default = nil)
-        unless default.nil?
-          ActiveSupport::Deprecation.warn(<<-MSG.squish)
-            Passing default to #index_name_exists? is deprecated without replacement.
-          MSG
-        end
+      def index_name_exists?(table_name, index_name)
         index_name = index_name.to_s
         indexes(table_name).detect { |i| i.name == index_name }
       end
 
-      # Adds a reference. The reference column is an integer by default,
+      # Adds a reference. The reference column is a bigint by default,
       # the <tt>:type</tt> option can be used to specify a different type.
       # Optionally adds a +_type+ column, if <tt>:polymorphic</tt> option is provided.
       # #add_reference and #add_belongs_to are acceptable.
       #
       # The +options+ hash can include the following keys:
       # [<tt>:type</tt>]
-      #   The reference column type. Defaults to +:integer+.
+      #   The reference column type. Defaults to +:bigint+.
       # [<tt>:index</tt>]
       #   Add an appropriate index. Defaults to true.
       #   See #add_index for usage of this option.
@@ -832,7 +851,7 @@ module ActiveRecord
       # [<tt>:null</tt>]
       #   Whether the column allows nulls. Defaults to true.
       #
-      # ====== Create a user_id integer column
+      # ====== Create a user_id bigint column
       #
       #   add_reference(:products, :user)
       #
@@ -945,6 +964,8 @@ module ActiveRecord
       #   Action that happens <tt>ON DELETE</tt>. Valid values are +:nullify+, +:cascade+ and +:restrict+
       # [<tt>:on_update</tt>]
       #   Action that happens <tt>ON UPDATE</tt>. Valid values are +:nullify+, +:cascade+ and +:restrict+
+      # [<tt>:validate</tt>]
+      #   (Postgres only) Specify whether or not the constraint should be validated. Defaults to +true+.
       def add_foreign_key(from_table, to_table, options = {})
         return unless supports_foreign_keys?
 
@@ -1018,16 +1039,6 @@ module ActiveRecord
         insert_versions_sql(versions) if versions.any?
       end
 
-      def initialize_schema_migrations_table # :nodoc:
-        ActiveRecord::SchemaMigration.create_table
-      end
-      deprecate :initialize_schema_migrations_table
-
-      def initialize_internal_metadata_table # :nodoc:
-        ActiveRecord::InternalMetadata.create_table
-      end
-      deprecate :initialize_internal_metadata_table
-
       def internal_string_options_for_primary_key # :nodoc:
         { primary_key: true }
       end
@@ -1133,7 +1144,7 @@ module ActiveRecord
       def add_index_options(table_name, column_name, comment: nil, **options) # :nodoc:
         column_names = index_column_names(column_name)
 
-        options.assert_valid_keys(:unique, :order, :name, :where, :length, :internal, :using, :algorithm, :type)
+        options.assert_valid_keys(:unique, :order, :name, :where, :length, :internal, :using, :algorithm, :type, :opclass)
 
         index_type = options[:type].to_s if options.key?(:type)
         index_type ||= options[:unique] ? "UNIQUE" : ""
@@ -1172,7 +1183,7 @@ module ActiveRecord
       end
 
       # Changes the comment for a column or removes it if +nil+.
-      def change_column_comment(table_name, column_name, comment) #:nodoc:
+      def change_column_comment(table_name, column_name, comment)
         raise NotImplementedError, "#{self.class} does not support changing column comments"
       end
 
@@ -1186,20 +1197,22 @@ module ActiveRecord
         end
 
         def add_index_sort_order(quoted_columns, **options)
-          if order = options[:order]
-            case order
-            when Hash
-              order = order.symbolize_keys
-              quoted_columns.each { |name, column| column << " #{order[name].upcase}" if order[name].present? }
-            when String
-              quoted_columns.each { |name, column| column << " #{order.upcase}" if order.present? }
-            end
+          orders = options_for_index_columns(options[:order])
+          quoted_columns.each do |name, column|
+            column << " #{orders[name].upcase}" if orders[name].present?
           end
+        end
 
-          quoted_columns
+        def options_for_index_columns(options)
+          if options.is_a?(Hash)
+            options.symbolize_keys
+          else
+            Hash.new { |hash, column| hash[column] = options }
+          end
         end
 
-        # Overridden by the MySQL adapter for supporting index lengths
+        # Overridden by the MySQL adapter for supporting index lengths and by
+        # the PostgreSQL adapter for supporting operator classes.
         def add_options_for_index_columns(quoted_columns, **options)
           if supports_index_sort_order?
             quoted_columns = add_index_sort_order(quoted_columns, options)
@@ -1353,6 +1366,20 @@ module ActiveRecord
           options.is_a?(Hash) && options.key?(:name) && options.except(:name, :algorithm).empty?
         end
 
+        def add_column_for_alter(table_name, column_name, type, options = {})
+          td = create_table_definition(table_name)
+          cd = td.new_column_definition(column_name, type, options)
+          schema_creation.accept(AddColumnDefinition.new(cd))
+        end
+
+        def remove_column_for_alter(table_name, column_name, type = nil, options = {})
+          "DROP COLUMN #{quote_column_name(column_name)}"
+        end
+
+        def remove_columns_for_alter(table_name, *column_names)
+          column_names.map { |column_name| remove_column_for_alter(table_name, column_name) }
+        end
+
         def insert_versions_sql(versions)
           sm_table = quote_table_name(ActiveRecord::SchemaMigration.table_name)
 
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb b/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb
index 147e16e9fa..d9ac8db6a8 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb
@@ -240,7 +240,7 @@ module ActiveRecord
                 rollback_transaction if transaction
               else
                 begin
-                  commit_transaction
+                  commit_transaction if transaction
                 rescue Exception
                   rollback_transaction(transaction) unless transaction.state.completed?
                   raise
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index 8c889f98f5..fc80d332f9 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
-require_relative "../type"
-require_relative "determine_if_preparable_visitor"
-require_relative "schema_cache"
-require_relative "sql_type_metadata"
-require_relative "abstract/schema_dumper"
-require_relative "abstract/schema_creation"
+require "active_record/connection_adapters/determine_if_preparable_visitor"
+require "active_record/connection_adapters/schema_cache"
+require "active_record/connection_adapters/sql_type_metadata"
+require "active_record/connection_adapters/abstract/schema_dumper"
+require "active_record/connection_adapters/abstract/schema_creation"
+require "active_support/concurrency/load_interlock_aware_monitor"
 require "arel/collectors/bind"
 require "arel/collectors/composite"
 require "arel/collectors/sql_string"
@@ -105,10 +105,11 @@ module ActiveRecord
         @logger              = logger
         @config              = config
         @pool                = nil
+        @idle_since          = Concurrent.monotonic_time
         @schema_cache        = SchemaCache.new self
         @quoted_column_names, @quoted_table_names = {}, {}
         @visitor = arel_visitor
-        @lock = Monitor.new
+        @lock = ActiveSupport::Concurrency::LoadInterlockAwareMonitor.new
 
         if self.class.type_cast_config_to_boolean(config.fetch(:prepared_statements) { true })
           @prepared_statements = true
@@ -164,6 +165,7 @@ module ActiveRecord
               "Current thread: #{Thread.current}."
           end
 
+          @idle_since = Concurrent.monotonic_time
           @owner = nil
         else
           raise ActiveRecordError, "Cannot expire connection, it is not currently leased."
@@ -183,6 +185,12 @@ module ActiveRecord
         end
       end
 
+      # Seconds since this connection was returned to the pool
+      def seconds_idle # :nodoc:
+        return 0 if in_use?
+        Concurrent.monotonic_time - @idle_since
+      end
+
       def unprepared_statement
         old_prepared_statements, @prepared_statements = @prepared_statements, false
         yield
@@ -196,16 +204,6 @@ module ActiveRecord
         self.class::ADAPTER_NAME
       end
 
-      def supports_migrations? # :nodoc:
-        true
-      end
-      deprecate :supports_migrations?
-
-      def supports_primary_key? # :nodoc:
-        true
-      end
-      deprecate :supports_primary_key?
-
       # Does this adapter support DDL rollbacks in transactions? That is, would
       # CREATE TABLE or ALTER TABLE get rolled back by a transaction?
       def supports_ddl_transactions?
@@ -274,6 +272,11 @@ module ActiveRecord
         false
       end
 
+      # Does this adapter support creating invalid constraints?
+      def supports_validate_constraints?
+        false
+      end
+
       # Does this adapter support creating foreign key constraints
       # in the same statement as creating the table?
       def supports_foreign_keys_in_create?
@@ -377,6 +380,19 @@ module ActiveRecord
         reset_transaction
       end
 
+      # Immediately forget this connection ever existed. Unlike disconnect!,
+      # this will not communicate with the server.
+      #
+      # After calling this method, the behavior of all other methods becomes
+      # undefined. This is called internally just before a forked process gets
+      # rid of a connection that belonged to its parent.
+      def discard!
+        # This should be overridden by concrete adapters.
+        #
+        # Prevent @connection's finalizer from touching the socket, or
+        # otherwise communicating with its server, when it is collected.
+      end
+
       # Reset the state of this connection, directing the DBMS to clear
       # transactions and other connection-related server-side state. Usually a
       # database-dependent operation.
@@ -402,10 +418,7 @@ module ActiveRecord
       # Checks whether the connection to the database is still active (i.e. not stale).
       # This is done under the hood by calling #active?. If the connection
       # is no longer active, then this method will reconnect to the database.
-      def verify!(*ignored)
-        if ignored.size > 0
-          ActiveSupport::Deprecation.warn("Passing arguments to #verify method of the connection has no effect and has been deprecated. Please remove all arguments from the #verify method call.")
-        end
+      def verify!
         reconnect! unless active?
       end
 
@@ -478,6 +491,8 @@ module ActiveRecord
           m.alias_type %r(number)i,    "decimal"
           m.alias_type %r(double)i,    "float"
 
+          m.register_type %r(^json)i, Type::Json.new
+
           m.register_type(%r(decimal)i) do |sql_type|
             scale = extract_scale(sql_type)
             precision = extract_precision(sql_type)
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 7cd086084a..479131caad 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -1,15 +1,15 @@
 # frozen_string_literal: true
 
-require_relative "abstract_adapter"
-require_relative "statement_pool"
-require_relative "mysql/column"
-require_relative "mysql/explain_pretty_printer"
-require_relative "mysql/quoting"
-require_relative "mysql/schema_creation"
-require_relative "mysql/schema_definitions"
-require_relative "mysql/schema_dumper"
-require_relative "mysql/schema_statements"
-require_relative "mysql/type_metadata"
+require "active_record/connection_adapters/abstract_adapter"
+require "active_record/connection_adapters/statement_pool"
+require "active_record/connection_adapters/mysql/column"
+require "active_record/connection_adapters/mysql/explain_pretty_printer"
+require "active_record/connection_adapters/mysql/quoting"
+require "active_record/connection_adapters/mysql/schema_creation"
+require "active_record/connection_adapters/mysql/schema_definitions"
+require "active_record/connection_adapters/mysql/schema_dumper"
+require "active_record/connection_adapters/mysql/schema_statements"
+require "active_record/connection_adapters/mysql/type_metadata"
 
 require "active_support/core_ext/string/strip"
 
@@ -44,7 +44,7 @@ module ActiveRecord
         json:        { name: "json" },
       }
 
-      class StatementPool < ConnectionAdapters::StatementPool
+      class StatementPool < ConnectionAdapters::StatementPool # :nodoc:
         private def dealloc(stmt)
           stmt[:stmt].close
         end
@@ -284,7 +284,7 @@ module ActiveRecord
       def table_comment(table_name) # :nodoc:
         scope = quoted_scope(table_name)
 
-        query_value(<<-SQL.strip_heredoc, "SCHEMA")
+        query_value(<<-SQL.strip_heredoc, "SCHEMA").presence
           SELECT table_comment
           FROM information_schema.tables
           WHERE table_schema = #{scope[:schema]}
@@ -299,7 +299,7 @@ module ActiveRecord
       def bulk_change_table(table_name, operations) #:nodoc:
         sqls = operations.flat_map do |command, args|
           table, arguments = args.shift, args
-          method = :"#{command}_sql"
+          method = :"#{command}_for_alter"
 
           if respond_to?(method, true)
             send(method, table, *arguments)
@@ -311,6 +311,11 @@ module ActiveRecord
         execute("ALTER TABLE #{quote_table_name(table_name)} #{sqls}")
       end
 
+      def change_table_comment(table_name, comment) #:nodoc:
+        comment = "" if comment.nil?
+        execute("ALTER TABLE #{quote_table_name(table_name)} COMMENT #{quote(comment)}")
+      end
+
       # Renames a table.
       #
       # Example:
@@ -351,26 +356,27 @@ module ActiveRecord
 
       def change_column_default(table_name, column_name, default_or_changes) #:nodoc:
         default = extract_new_default_value(default_or_changes)
-        column = column_for(table_name, column_name)
-        change_column table_name, column_name, column.sql_type, default: default
+        change_column table_name, column_name, nil, default: default
       end
 
       def change_column_null(table_name, column_name, null, default = nil) #:nodoc:
-        column = column_for(table_name, column_name)
-
         unless null || default.nil?
           execute("UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote(default)} WHERE #{quote_column_name(column_name)} IS NULL")
         end
 
-        change_column table_name, column_name, column.sql_type, null: null
+        change_column table_name, column_name, nil, null: null
+      end
+
+      def change_column_comment(table_name, column_name, comment) #:nodoc:
+        change_column table_name, column_name, nil, comment: comment
       end
 
       def change_column(table_name, column_name, type, options = {}) #:nodoc:
-        execute("ALTER TABLE #{quote_table_name(table_name)} #{change_column_sql(table_name, column_name, type, options)}")
+        execute("ALTER TABLE #{quote_table_name(table_name)} #{change_column_for_alter(table_name, column_name, type, options)}")
       end
 
       def rename_column(table_name, column_name, new_column_name) #:nodoc:
-        execute("ALTER TABLE #{quote_table_name(table_name)} #{rename_column_sql(table_name, column_name, new_column_name)}")
+        execute("ALTER TABLE #{quote_table_name(table_name)} #{rename_column_for_alter(table_name, column_name, new_column_name)}")
         rename_column_indexes(table_name, column_name, new_column_name)
       end
 
@@ -557,7 +563,6 @@ module ActiveRecord
           m.register_type %r(longblob)i,   Type::Binary.new(limit: 2**32 - 1)
           m.register_type %r(^float)i,     Type::Float.new(limit: 24)
           m.register_type %r(^double)i,    Type::Float.new(limit: 53)
-          m.register_type %r(^json)i,      Type::Json.new
 
           register_integer_type m, %r(^bigint)i,    limit: 8
           register_integer_type m, %r(^int)i,       limit: 4
@@ -600,25 +605,6 @@ module ActiveRecord
           end
         end
 
-        def add_index_length(quoted_columns, **options)
-          if length = options[:length]
-            case length
-            when Hash
-              length = length.symbolize_keys
-              quoted_columns.each { |name, column| column << "(#{length[name]})" if length[name].present? }
-            when Integer
-              quoted_columns.each { |name, column| column << "(#{length})" }
-            end
-          end
-
-          quoted_columns
-        end
-
-        def add_options_for_index_columns(quoted_columns, **options)
-          quoted_columns = add_index_length(quoted_columns, options)
-          super
-        end
-
         # See https://dev.mysql.com/doc/refman/5.7/en/error-messages-server.html
         ER_DUP_ENTRY            = 1062
         ER_NOT_NULL_VIOLATION   = 1048
@@ -630,6 +616,8 @@ module ActiveRecord
         ER_CANNOT_ADD_FOREIGN   = 1215
         ER_CANNOT_CREATE_TABLE  = 1005
         ER_LOCK_WAIT_TIMEOUT    = 1205
+        ER_QUERY_INTERRUPTED    = 1317
+        ER_QUERY_TIMEOUT        = 3024
 
         def translate_exception(exception, message)
           case error_number(exception)
@@ -654,20 +642,19 @@ module ActiveRecord
           when ER_LOCK_DEADLOCK
             Deadlocked.new(message)
           when ER_LOCK_WAIT_TIMEOUT
-            TransactionTimeout.new(message)
+            LockWaitTimeout.new(message)
+          when ER_QUERY_TIMEOUT
+            StatementTimeout.new(message)
+          when ER_QUERY_INTERRUPTED
+            QueryCanceled.new(message)
           else
             super
           end
         end
 
-        def add_column_sql(table_name, column_name, type, options = {})
-          td = create_table_definition(table_name)
-          cd = td.new_column_definition(column_name, type, options)
-          schema_creation.accept(AddColumnDefinition.new(cd))
-        end
-
-        def change_column_sql(table_name, column_name, type, options = {})
+        def change_column_for_alter(table_name, column_name, type, options = {})
           column = column_for(table_name, column_name)
+          type ||= column.sql_type
 
           unless options.key?(:default)
             options[:default] = column.default
@@ -686,7 +673,7 @@ module ActiveRecord
           schema_creation.accept(ChangeColumnDefinition.new(cd, column.name))
         end
 
-        def rename_column_sql(table_name, column_name, new_column_name)
+        def rename_column_for_alter(table_name, column_name, new_column_name)
           column  = column_for(table_name, column_name)
           options = {
             default: column.default,
@@ -700,31 +687,23 @@ module ActiveRecord
           schema_creation.accept(ChangeColumnDefinition.new(cd, column.name))
         end
 
-        def remove_column_sql(table_name, column_name, type = nil, options = {})
-          "DROP #{quote_column_name(column_name)}"
-        end
-
-        def remove_columns_sql(table_name, *column_names)
-          column_names.map { |column_name| remove_column_sql(table_name, column_name) }
-        end
-
-        def add_index_sql(table_name, column_name, options = {})
+        def add_index_for_alter(table_name, column_name, options = {})
           index_name, index_type, index_columns, _, index_algorithm, index_using = add_index_options(table_name, column_name, options)
           index_algorithm[0, 0] = ", " if index_algorithm.present?
           "ADD #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})#{index_algorithm}"
         end
 
-        def remove_index_sql(table_name, options = {})
+        def remove_index_for_alter(table_name, options = {})
           index_name = index_name_for_remove(table_name, options)
-          "DROP INDEX #{index_name}"
+          "DROP INDEX #{quote_column_name(index_name)}"
         end
 
-        def add_timestamps_sql(table_name, options = {})
-          [add_column_sql(table_name, :created_at, :datetime, options), add_column_sql(table_name, :updated_at, :datetime, options)]
+        def add_timestamps_for_alter(table_name, options = {})
+          [add_column_for_alter(table_name, :created_at, :datetime, options), add_column_for_alter(table_name, :updated_at, :datetime, options)]
         end
 
-        def remove_timestamps_sql(table_name, options = {})
-          [remove_column_sql(table_name, :updated_at), remove_column_sql(table_name, :created_at)]
+        def remove_timestamps_for_alter(table_name, options = {})
+          [remove_column_for_alter(table_name, :updated_at), remove_column_for_alter(table_name, :created_at)]
         end
 
         # MySQL is too stupid to create a temporary table for use subquery, so we have
diff --git a/activerecord/lib/active_record/connection_adapters/column.rb b/activerecord/lib/active_record/connection_adapters/column.rb
index 16273fb5f1..5d81de9fe1 100644
--- a/activerecord/lib/active_record/connection_adapters/column.rb
+++ b/activerecord/lib/active_record/connection_adapters/column.rb
@@ -11,11 +11,11 @@ module ActiveRecord
 
       # Instantiates a new column in the table.
       #
-      # +name+ is the column's name, such as <tt>supplier_id</tt> in <tt>supplier_id int</tt>.
+      # +name+ is the column's name, such as <tt>supplier_id</tt> in <tt>supplier_id bigint</tt>.
       # +default+ is the type-casted default value, such as +new+ in <tt>sales_stage varchar(20) default 'new'</tt>.
       # +sql_type_metadata+ is various information about the type of the column
       # +null+ determines if this column allows +NULL+ values.
-      def initialize(name, default, sql_type_metadata = nil, null = true, table_name = nil, default_function = nil, collation = nil, comment: nil)
+      def initialize(name, default, sql_type_metadata = nil, null = true, table_name = nil, default_function = nil, collation = nil, comment: nil, **)
         @name = name.freeze
         @table_name = table_name
         @sql_type_metadata = sql_type_metadata
diff --git a/activerecord/lib/active_record/connection_adapters/connection_specification.rb b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
index 29542f917e..508132accb 100644
--- a/activerecord/lib/active_record/connection_adapters/connection_specification.rb
+++ b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
@@ -183,13 +183,25 @@ module ActiveRecord
 
           raise(AdapterNotSpecified, "database configuration does not specify adapter") unless spec.key?(:adapter)
 
+          # Require the adapter itself and give useful feedback about
+          #   1. Missing adapter gems and
+          #   2. Adapter gems' missing dependencies.
           path_to_adapter = "active_record/connection_adapters/#{spec[:adapter]}_adapter"
           begin
             require path_to_adapter
-          rescue Gem::LoadError => e
-            raise Gem::LoadError, "Specified '#{spec[:adapter]}' for database adapter, but the gem is not loaded. Add `gem '#{e.name}'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord)."
           rescue LoadError => e
-            raise LoadError, "Could not load '#{path_to_adapter}'. Make sure that the adapter in config/database.yml is valid. If you use an adapter other than 'mysql2', 'postgresql' or 'sqlite3' add the necessary adapter gem to the Gemfile.", e.backtrace
+            # We couldn't require the adapter itself. Raise an exception that
+            # points out config typos and missing gems.
+            if e.path == path_to_adapter
+              # We can assume that a non-builtin adapter was specified, so it's
+              # either misspelled or missing from Gemfile.
+              raise e.class, "Could not load the '#{spec[:adapter]}' Active Record adapter. Ensure that the adapter is spelled correctly in config/database.yml and that you've added the necessary adapter gem to your Gemfile.", e.backtrace
+
+            # Bubbled up from the adapter require. Prefix the exception message
+            # with some guidance about how to address it and reraise.
+            else
+              raise e.class, "Error loading the '#{spec[:adapter]}' Active Record adapter. Missing a gem it depends on? #{e.message}", e.backtrace
+            end
           end
 
           adapter_method = "#{spec[:adapter]}_connection"
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb
index b22a2e4da7..2ed4ad16ae 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb
@@ -4,11 +4,6 @@ module ActiveRecord
   module ConnectionAdapters
     module MySQL
       module ColumnMethods
-        def primary_key(name, type = :primary_key, **options)
-          options[:auto_increment] = true if [:integer, :bigint].include?(type) && !options.key?(:default)
-          super
-        end
-
         def blob(*args, **options)
           args.each { |name| column(name, :blob, options) }
         end
@@ -37,10 +32,6 @@ module ActiveRecord
           args.each { |name| column(name, :longtext, options) }
         end
 
-        def json(*args, **options)
-          args.each { |name| column(name, :json, options) }
-        end
-
         def unsigned_integer(*args, **options)
           args.each { |name| column(name, :unsigned_integer, options) }
         end
@@ -68,7 +59,6 @@ module ActiveRecord
           when :primary_key
             type = :integer
             options[:limit] ||= 8
-            options[:auto_increment] = true
             options[:primary_key] = true
           when /\Aunsigned_(?<type>.+)\z/
             type = $~[:type].to_sym
@@ -82,6 +72,11 @@ module ActiveRecord
           def aliased_types(name, fallback)
             fallback
           end
+
+          def integer_like_primary_key_type(type, options)
+            options[:auto_increment] = true
+            type
+          end
       end
 
       class Table < ActiveRecord::ConnectionAdapters::Table
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb
index 95eb77aea4..d23178e43c 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb
@@ -8,6 +8,7 @@ module ActiveRecord
           def prepare_column_options(column)
             spec = super
             spec[:unsigned] = "true" if column.unsigned?
+            spec[:auto_increment] = "true" if column.auto_increment?
 
             if @connection.supports_virtual_columns? && column.virtual?
               spec[:as] = extract_expression_for_virtual_column(column)
@@ -18,6 +19,12 @@ module ActiveRecord
             spec
           end
 
+          def column_spec_for_primary_key(column)
+            spec = super
+            spec.delete(:auto_increment) if column.type == :integer && column.auto_increment?
+            spec
+          end
+
           def default_primary_key?(column)
             super && column.auto_increment? && !column.unsigned?
           end
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
index 759493e3bd..ce50590651 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
@@ -5,13 +5,7 @@ module ActiveRecord
     module MySQL
       module SchemaStatements # :nodoc:
         # Returns an array of indexes for the given table.
-        def indexes(table_name, name = nil)
-          if name
-            ActiveSupport::Deprecation.warn(<<-MSG.squish)
-              Passing name to #indexes is deprecated without replacement.
-            MSG
-          end
-
+        def indexes(table_name)
           indexes = []
           current_index = nil
           execute_and_free("SHOW KEYS FROM #{quote_table_name(table_name)}", "SCHEMA") do |result|
@@ -28,23 +22,26 @@ module ActiveRecord
                   index_using = mysql_index_type
                 end
 
-                indexes << IndexDefinition.new(
+                indexes << [
                   row[:Table],
                   row[:Key_name],
                   row[:Non_unique].to_i == 0,
+                  [],
+                  lengths: {},
+                  orders: {},
                   type: index_type,
                   using: index_using,
                   comment: row[:Index_comment].presence
-                )
+                ]
               end
 
-              indexes.last.columns << row[:Column_name]
-              indexes.last.lengths.merge!(row[:Column_name] => row[:Sub_part].to_i) if row[:Sub_part]
-              indexes.last.orders.merge!(row[:Column_name] => :desc) if row[:Collation] == "D"
+              indexes.last[-2] << row[:Column_name]
+              indexes.last[-1][:lengths].merge!(row[:Column_name] => row[:Sub_part].to_i) if row[:Sub_part]
+              indexes.last[-1][:orders].merge!(row[:Column_name] => :desc) if row[:Collation] == "D"
             end
           end
 
-          indexes
+          indexes.map { |index| IndexDefinition.new(*index) }
         end
 
         def remove_column(table_name, column_name, type = nil, options = {})
@@ -109,6 +106,18 @@ module ActiveRecord
             super unless specifier == "RESTRICT"
           end
 
+          def add_index_length(quoted_columns, **options)
+            lengths = options_for_index_columns(options[:length])
+            quoted_columns.each do |name, column|
+              column << "(#{lengths[name]})" if lengths[name].present?
+            end
+          end
+
+          def add_options_for_index_columns(quoted_columns, **options)
+            quoted_columns = add_index_length(quoted_columns, options)
+            super
+          end
+
           def data_source_sql(name = nil, type: nil)
             scope = quoted_scope(name, type: type)
 
diff --git a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
index 2c2321872d..bfdc7995f0 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
@@ -1,11 +1,10 @@
 # frozen_string_literal: true
 
-require_relative "abstract_mysql_adapter"
-require_relative "mysql/database_statements"
+require "active_record/connection_adapters/abstract_mysql_adapter"
+require "active_record/connection_adapters/mysql/database_statements"
 
-gem "mysql2", ">= 0.3.18", "< 0.5"
+gem "mysql2", "~> 0.4.4"
 require "mysql2"
-raise "mysql2 0.4.3 is not supported. Please upgrade to 0.4.4+" if Mysql2::VERSION == "0.4.3"
 
 module ActiveRecord
   module ConnectionHandling # :nodoc:
@@ -105,6 +104,11 @@ module ActiveRecord
         @connection.close
       end
 
+      def discard! # :nodoc:
+        @connection.automatic_close = false
+        @connection = nil
+      end
+
       private
 
         def connect
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/column.rb b/activerecord/lib/active_record/connection_adapters/postgresql/column.rb
index 1b67cee24b..469ef3f5a0 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/column.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/column.rb
@@ -7,11 +7,38 @@ module ActiveRecord
       delegate :array, :oid, :fmod, to: :sql_type_metadata
       alias :array? :array
 
+      def initialize(*, max_identifier_length: 63, **)
+        super
+        @max_identifier_length = max_identifier_length
+      end
+
       def serial?
         return unless default_function
 
-        %r{\Anextval\('"?#{table_name}_#{name}_seq"?'::regclass\)\z} === default_function
+        if %r{\Anextval\('"?(?<sequence_name>.+_(?<suffix>seq\d*))"?'::regclass\)\z} =~ default_function
+          sequence_name_from_parts(table_name, name, suffix) == sequence_name
+        end
       end
+
+      protected
+        attr_reader :max_identifier_length
+
+      private
+        def sequence_name_from_parts(table_name, column_name, suffix)
+          over_length = [table_name, column_name, suffix].map(&:length).sum + 2 - max_identifier_length
+
+          if over_length > 0
+            column_name_length = [(max_identifier_length - suffix.length - 2) / 2, column_name.length].min
+            over_length -= column_name.length - column_name_length
+            column_name = column_name[0, column_name_length - [over_length, 0].min]
+          end
+
+          if over_length > 0
+            table_name = table_name[0, table_name.length - over_length]
+          end
+
+          "#{table_name}_#{column_name}_#{suffix}"
+        end
     end
   end
 end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb
index b28418d74f..542ca75d3e 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb
@@ -1,27 +1,27 @@
 # frozen_string_literal: true
 
-require_relative "oid/array"
-require_relative "oid/bit"
-require_relative "oid/bit_varying"
-require_relative "oid/bytea"
-require_relative "oid/cidr"
-require_relative "oid/date_time"
-require_relative "oid/decimal"
-require_relative "oid/enum"
-require_relative "oid/hstore"
-require_relative "oid/inet"
-require_relative "oid/jsonb"
-require_relative "oid/money"
-require_relative "oid/oid"
-require_relative "oid/point"
-require_relative "oid/legacy_point"
-require_relative "oid/range"
-require_relative "oid/specialized_string"
-require_relative "oid/uuid"
-require_relative "oid/vector"
-require_relative "oid/xml"
+require "active_record/connection_adapters/postgresql/oid/array"
+require "active_record/connection_adapters/postgresql/oid/bit"
+require "active_record/connection_adapters/postgresql/oid/bit_varying"
+require "active_record/connection_adapters/postgresql/oid/bytea"
+require "active_record/connection_adapters/postgresql/oid/cidr"
+require "active_record/connection_adapters/postgresql/oid/date_time"
+require "active_record/connection_adapters/postgresql/oid/decimal"
+require "active_record/connection_adapters/postgresql/oid/enum"
+require "active_record/connection_adapters/postgresql/oid/hstore"
+require "active_record/connection_adapters/postgresql/oid/inet"
+require "active_record/connection_adapters/postgresql/oid/jsonb"
+require "active_record/connection_adapters/postgresql/oid/money"
+require "active_record/connection_adapters/postgresql/oid/oid"
+require "active_record/connection_adapters/postgresql/oid/point"
+require "active_record/connection_adapters/postgresql/oid/legacy_point"
+require "active_record/connection_adapters/postgresql/oid/range"
+require "active_record/connection_adapters/postgresql/oid/specialized_string"
+require "active_record/connection_adapters/postgresql/oid/uuid"
+require "active_record/connection_adapters/postgresql/oid/vector"
+require "active_record/connection_adapters/postgresql/oid/xml"
 
-require_relative "oid/type_map_initializer"
+require "active_record/connection_adapters/postgresql/oid/type_map_initializer"
 
 module ActiveRecord
   module ConnectionAdapters
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid/range.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid/range.rb
index 7d5d7d91e6..a89aa5ea09 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid/range.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid/range.rb
@@ -35,7 +35,7 @@ module ActiveRecord
             if value.is_a?(::Range)
               from = type_cast_single_for_database(value.begin)
               to = type_cast_single_for_database(value.end)
-              "[#{from},#{to}#{value.exclude_end? ? ')' : ']'}"
+              ::Range.new(from, to, value.exclude_end?)
             else
               super
             end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/quoting.rb b/activerecord/lib/active_record/connection_adapters/postgresql/quoting.rb
index a0a22ba0f1..9fdeab06c1 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/quoting.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/quoting.rb
@@ -101,6 +101,8 @@ module ActiveRecord
               end
             when OID::Array::Data
               _quote(encode_array(value))
+            when Range
+              _quote(encode_range(value))
             else
               super
             end
@@ -117,6 +119,8 @@ module ActiveRecord
               value.to_s
             when OID::Array::Data
               encode_array(value)
+            when Range
+              encode_range(value)
             else
               super
             end
@@ -133,6 +137,10 @@ module ActiveRecord
             result
           end
 
+          def encode_range(range)
+            "[#{type_cast(range.first)},#{type_cast(range.last)}#{range.exclude_end? ? ')' : ']'}"
+          end
+
           def determine_encoding_of_strings_in_array(value)
             case value
             when ::Array then determine_encoding_of_strings_in_array(value.first)
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/referential_integrity.rb b/activerecord/lib/active_record/connection_adapters/postgresql/referential_integrity.rb
index 386d22a9bd..8df91c988b 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/referential_integrity.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/referential_integrity.rb
@@ -4,26 +4,21 @@ module ActiveRecord
   module ConnectionAdapters
     module PostgreSQL
       module ReferentialIntegrity # :nodoc:
-        def supports_disable_referential_integrity? # :nodoc:
-          true
-        end
-
         def disable_referential_integrity # :nodoc:
-          if supports_disable_referential_integrity?
-            original_exception = nil
+          original_exception = nil
 
-            begin
-              transaction(requires_new: true) do
-                execute(tables.collect { |name| "ALTER TABLE #{quote_table_name(name)} DISABLE TRIGGER ALL" }.join(";"))
-              end
-            rescue ActiveRecord::ActiveRecordError => e
-              original_exception = e
+          begin
+            transaction(requires_new: true) do
+              execute(tables.collect { |name| "ALTER TABLE #{quote_table_name(name)} DISABLE TRIGGER ALL" }.join(";"))
             end
+          rescue ActiveRecord::ActiveRecordError => e
+            original_exception = e
+          end
 
-            begin
-              yield
-            rescue ActiveRecord::InvalidForeignKey => e
-              warn <<-WARNING
+          begin
+            yield
+          rescue ActiveRecord::InvalidForeignKey => e
+            warn <<-WARNING
 WARNING: Rails was not able to disable referential integrity.
 
 This is most likely caused due to missing permissions.
@@ -32,17 +27,14 @@ Rails needs superuser privileges to disable referential integrity.
     cause: #{original_exception.try(:message)}
 
               WARNING
-              raise e
-            end
+            raise e
+          end
 
-            begin
-              transaction(requires_new: true) do
-                execute(tables.collect { |name| "ALTER TABLE #{quote_table_name(name)} ENABLE TRIGGER ALL" }.join(";"))
-              end
-            rescue ActiveRecord::ActiveRecordError
+          begin
+            transaction(requires_new: true) do
+              execute(tables.collect { |name| "ALTER TABLE #{quote_table_name(name)} ENABLE TRIGGER ALL" }.join(";"))
             end
-          else
-            yield
+          rescue ActiveRecord::ActiveRecordError
           end
         end
       end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_creation.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_creation.rb
index 59f661da25..8e381a92cf 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_creation.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_creation.rb
@@ -5,6 +5,18 @@ module ActiveRecord
     module PostgreSQL
       class SchemaCreation < AbstractAdapter::SchemaCreation # :nodoc:
         private
+          def visit_AlterTable(o)
+            super << o.constraint_validations.map { |fk| visit_ValidateConstraint fk }.join(" ")
+          end
+
+          def visit_AddForeignKey(o)
+            super.dup.tap { |sql| sql << " NOT VALID" unless o.validate? }
+          end
+
+          def visit_ValidateConstraint(name)
+            "VALIDATE CONSTRAINT #{quote_column_name(name)}"
+          end
+
           def add_column_options!(sql, options)
             if options[:collation]
               sql << " COLLATE \"#{options[:collation]}\""
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_definitions.rb
index f1489e4d69..6047217fcd 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_definitions.rb
@@ -44,15 +44,8 @@ module ActiveRecord
         # a record (as primary keys cannot be +nil+). This might be done via the
         # +SecureRandom.uuid+ method and a +before_save+ callback, for instance.
         def primary_key(name, type = :primary_key, **options)
-          options[:auto_increment] = true if [:integer, :bigint].include?(type) && !options.key?(:default)
           if type == :uuid
             options[:default] = options.fetch(:default, "gen_random_uuid()")
-          elsif options.delete(:auto_increment) == true && %i(integer bigint).include?(type)
-            type = if type == :bigint || options[:limit] == 8
-              :bigserial
-            else
-              :serial
-            end
           end
 
           super
@@ -102,10 +95,6 @@ module ActiveRecord
           args.each { |name| column(name, :int8range, options) }
         end
 
-        def json(*args, **options)
-          args.each { |name| column(name, :json, options) }
-        end
-
         def jsonb(*args, **options)
           args.each { |name| column(name, :jsonb, options) }
         end
@@ -185,11 +174,33 @@ module ActiveRecord
 
       class TableDefinition < ActiveRecord::ConnectionAdapters::TableDefinition
         include ColumnMethods
+
+        private
+          def integer_like_primary_key_type(type, options)
+            if type == :bigint || options[:limit] == 8
+              :bigserial
+            else
+              :serial
+            end
+          end
       end
 
       class Table < ActiveRecord::ConnectionAdapters::Table
         include ColumnMethods
       end
+
+      class AlterTable < ActiveRecord::ConnectionAdapters::AlterTable
+        attr_reader :constraint_validations
+
+        def initialize(td)
+          super
+          @constraint_validations = []
+        end
+
+        def validate_constraint(name)
+          @constraint_validations << name
+        end
+      end
     end
   end
 end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_dumper.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_dumper.rb
index c0dbb166b7..84643d20da 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_dumper.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_dumper.rb
@@ -5,6 +5,18 @@ module ActiveRecord
     module PostgreSQL
       class SchemaDumper < ConnectionAdapters::SchemaDumper # :nodoc:
         private
+
+          def extensions(stream)
+            extensions = @connection.extensions
+            if extensions.any?
+              stream.puts "  # These are extensions that must be enabled in order to support this database"
+              extensions.sort.each do |extension|
+                stream.puts "  enable_extension #{extension.inspect}"
+              end
+              stream.puts
+            end
+          end
+
           def prepare_column_options(column)
             spec = super
             spec[:array] = "true" if column.array?
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 9e2f61e6ce..bf5fbb30e1 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "active_support/core_ext/string/strip"
-
 module ActiveRecord
   module ConnectionAdapters
     module PostgreSQL
@@ -66,12 +64,7 @@ module ActiveRecord
         end
 
         # Verifies existence of an index with a given name.
-        def index_name_exists?(table_name, index_name, default = nil)
-          unless default.nil?
-            ActiveSupport::Deprecation.warn(<<-MSG.squish)
-              Passing default to #index_name_exists? is deprecated without replacement.
-            MSG
-          end
+        def index_name_exists?(table_name, index_name)
           table = quoted_scope(table_name)
           index = quoted_scope(index_name)
 
@@ -89,21 +82,12 @@ module ActiveRecord
         end
 
         # Returns an array of indexes for the given table.
-        def indexes(table_name, name = nil) # :nodoc:
-          if name
-            ActiveSupport::Deprecation.warn(<<-MSG.squish)
-              Passing name to #indexes is deprecated without replacement.
-            MSG
-          end
-
+        def indexes(table_name) # :nodoc:
           scope = quoted_scope(table_name)
 
           result = query(<<-SQL, "SCHEMA")
             SELECT distinct i.relname, d.indisunique, d.indkey, pg_get_indexdef(d.indexrelid), t.oid,
-                            pg_catalog.obj_description(i.oid, 'pg_class') AS comment,
-            (SELECT COUNT(*) FROM pg_opclass o
-               JOIN (SELECT unnest(string_to_array(d.indclass::text, ' '))::int oid) c
-                 ON o.oid = c.oid WHERE o.opcdefault = 'f')
+                            pg_catalog.obj_description(i.oid, 'pg_class') AS comment
             FROM pg_class t
             INNER JOIN pg_index d ON t.oid = d.indrelid
             INNER JOIN pg_class i ON d.indexrelid = i.oid
@@ -122,11 +106,13 @@ module ActiveRecord
             inddef = row[3]
             oid = row[4]
             comment = row[5]
-            opclass = row[6]
 
             using, expressions, where = inddef.scan(/ USING (\w+?) \((.+?)\)(?: WHERE (.+))?\z/).flatten
 
-            if indkey.include?(0) || opclass > 0
+            orders = {}
+            opclasses = {}
+
+            if indkey.include?(0)
               columns = expressions
             else
               columns = Hash[query(<<-SQL.strip_heredoc, "SCHEMA")].values_at(*indkey).compact
@@ -136,10 +122,12 @@ module ActiveRecord
                 AND a.attnum IN (#{indkey.join(",")})
               SQL
 
-              # add info on sort order for columns (only desc order is explicitly specified, asc is the default)
-              orders = Hash[
-                expressions.scan(/(\w+) DESC/).flatten.map { |order_column| [order_column, :desc] }
-              ]
+              # add info on sort order (only desc order is explicitly specified, asc is the default)
+              # and non-default opclasses
+              expressions.scan(/(\w+)(?: (?!DESC)(\w+))?(?: (DESC))?/).each do |column, opclass, desc|
+                opclasses[column] = opclass.to_sym if opclass
+                orders[column] = :desc if desc
+              end
             end
 
             IndexDefinition.new(
@@ -148,6 +136,7 @@ module ActiveRecord
               unique,
               columns,
               orders: orders,
+              opclasses: opclasses,
               where: where,
               using: using.to_sym,
               comment: comment.presence
@@ -405,50 +394,23 @@ module ActiveRecord
 
         def change_column(table_name, column_name, type, options = {}) #:nodoc:
           clear_cache!
-          quoted_table_name = quote_table_name(table_name)
-          quoted_column_name = quote_column_name(column_name)
-          sql_type = type_to_sql(type, options)
-          sql = "ALTER TABLE #{quoted_table_name} ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}".dup
-          if options[:collation]
-            sql << " COLLATE \"#{options[:collation]}\""
-          end
-          if options[:using]
-            sql << " USING #{options[:using]}"
-          elsif options[:cast_as]
-            cast_as_type = type_to_sql(options[:cast_as], options)
-            sql << " USING CAST(#{quoted_column_name} AS #{cast_as_type})"
-          end
-          execute sql
-
-          change_column_default(table_name, column_name, options[:default]) if options.key?(:default)
-          change_column_null(table_name, column_name, options[:null], options[:default]) if options.key?(:null)
-          change_column_comment(table_name, column_name, options[:comment]) if options.key?(:comment)
+          sqls, procs = change_column_for_alter(table_name, column_name, type, options)
+          execute "ALTER TABLE #{quote_table_name(table_name)} #{sqls.join(", ")}"
+          procs.each(&:call)
         end
 
         # Changes the default value of a table column.
         def change_column_default(table_name, column_name, default_or_changes) # :nodoc:
-          clear_cache!
-          column = column_for(table_name, column_name)
-          return unless column
-
-          default = extract_new_default_value(default_or_changes)
-          alter_column_query = "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} %s"
-          if default.nil?
-            # <tt>DEFAULT NULL</tt> results in the same behavior as <tt>DROP DEFAULT</tt>. However, PostgreSQL will
-            # cast the default to the columns type, which leaves us with a default like "default NULL::character varying".
-            execute alter_column_query % "DROP DEFAULT"
-          else
-            execute alter_column_query % "SET DEFAULT #{quote_default_expression(default, column)}"
-          end
+          execute "ALTER TABLE #{quote_table_name(table_name)} #{change_column_default_for_alter(table_name, column_name, default_or_changes)}"
         end
 
         def change_column_null(table_name, column_name, null, default = nil) #:nodoc:
           clear_cache!
           unless null || default.nil?
             column = column_for(table_name, column_name)
-            execute("UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote_default_expression(default, column)} WHERE #{quote_column_name(column_name)} IS NULL") if column
+            execute "UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote_default_expression(default, column)} WHERE #{quote_column_name(column_name)} IS NULL" if column
           end
-          execute("ALTER TABLE #{quote_table_name(table_name)} ALTER #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL")
+          execute "ALTER TABLE #{quote_table_name(table_name)} #{change_column_null_for_alter(table_name, column_name, null, default)}"
         end
 
         # Adds comment for given table column or drops it if +comment+ is a +nil+
@@ -471,8 +433,8 @@ module ActiveRecord
         end
 
         def add_index(table_name, column_name, options = {}) #:nodoc:
-          index_name, index_type, index_columns, index_options, index_algorithm, index_using, comment = add_index_options(table_name, column_name, options)
-          execute("CREATE #{index_type} INDEX #{index_algorithm} #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} #{index_using} (#{index_columns})#{index_options}").tap do
+          index_name, index_type, index_columns_and_opclasses, index_options, index_algorithm, index_using, comment = add_index_options(table_name, column_name, options)
+          execute("CREATE #{index_type} INDEX #{index_algorithm} #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} #{index_using} (#{index_columns_and_opclasses})#{index_options}").tap do
             execute "COMMENT ON INDEX #{quote_column_name(index_name)} IS #{quote(comment)}" if comment
           end
         end
@@ -512,7 +474,7 @@ module ActiveRecord
         def foreign_keys(table_name)
           scope = quoted_scope(table_name)
           fk_info = exec_query(<<-SQL.strip_heredoc, "SCHEMA")
-            SELECT t2.oid::regclass::text AS to_table, a1.attname AS column, a2.attname AS primary_key, c.conname AS name, c.confupdtype AS on_update, c.confdeltype AS on_delete
+            SELECT t2.oid::regclass::text AS to_table, a1.attname AS column, a2.attname AS primary_key, c.conname AS name, c.confupdtype AS on_update, c.confdeltype AS on_delete, c.convalidated AS valid
             FROM pg_constraint c
             JOIN pg_class t1 ON c.conrelid = t1.oid
             JOIN pg_class t2 ON c.confrelid = t2.oid
@@ -534,6 +496,7 @@ module ActiveRecord
 
             options[:on_delete] = extract_foreign_key_action(row["on_delete"])
             options[:on_update] = extract_foreign_key_action(row["on_update"])
+            options[:validate] = row["valid"]
 
             ForeignKeyDefinition.new(table_name, row["to_table"], options)
           end
@@ -594,6 +557,43 @@ module ActiveRecord
           PostgreSQL::SchemaDumper.create(self, options)
         end
 
+        # Validates the given constraint.
+        #
+        # Validates the constraint named +constraint_name+ on +accounts+.
+        #
+        #   validate_constraint :accounts, :constraint_name
+        def validate_constraint(table_name, constraint_name)
+          return unless supports_validate_constraints?
+
+          at = create_alter_table table_name
+          at.validate_constraint constraint_name
+
+          execute schema_creation.accept(at)
+        end
+
+        # Validates the given foreign key.
+        #
+        # Validates the foreign key on +accounts.branch_id+.
+        #
+        #   validate_foreign_key :accounts, :branches
+        #
+        # Validates the foreign key on +accounts.owner_id+.
+        #
+        #   validate_foreign_key :accounts, column: :owner_id
+        #
+        # Validates the foreign key named +special_fk_name+ on the +accounts+ table.
+        #
+        #   validate_foreign_key :accounts, name: :special_fk_name
+        #
+        # The +options+ hash accepts the same keys as SchemaStatements#add_foreign_key.
+        def validate_foreign_key(from_table, options_or_to_table = {})
+          return unless supports_validate_constraints?
+
+          fk_name_to_validate = foreign_key_for!(from_table, options_or_to_table).name
+
+          validate_constraint from_table, fk_name_to_validate
+        end
+
         private
           def schema_creation
             PostgreSQL::SchemaCreation.new(self)
@@ -603,6 +603,10 @@ module ActiveRecord
             PostgreSQL::TableDefinition.new(*args)
           end
 
+          def create_alter_table(name)
+            PostgreSQL::AlterTable.new create_table_definition(name)
+          end
+
           def new_column_from_field(table_name, field)
             column_name, type, default, notnull, oid, fmod, collation, comment = field
             type_metadata = fetch_type_metadata(column_name, type, oid.to_i, fmod.to_i)
@@ -617,7 +621,8 @@ module ActiveRecord
               table_name,
               default_function,
               collation,
-              comment: comment.presence
+              comment: comment.presence,
+              max_identifier_length: max_identifier_length
             )
           end
 
@@ -641,6 +646,66 @@ module ActiveRecord
             end
           end
 
+          def change_column_sql(table_name, column_name, type, options = {})
+            quoted_column_name = quote_column_name(column_name)
+            sql_type = type_to_sql(type, options)
+            sql = "ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}".dup
+            if options[:collation]
+              sql << " COLLATE \"#{options[:collation]}\""
+            end
+            if options[:using]
+              sql << " USING #{options[:using]}"
+            elsif options[:cast_as]
+              cast_as_type = type_to_sql(options[:cast_as], options)
+              sql << " USING CAST(#{quoted_column_name} AS #{cast_as_type})"
+            end
+
+            sql
+          end
+
+          def change_column_for_alter(table_name, column_name, type, options = {})
+            sqls = [change_column_sql(table_name, column_name, type, options)]
+            procs = []
+            sqls << change_column_default_for_alter(table_name, column_name, options[:default]) if options.key?(:default)
+            sqls << change_column_null_for_alter(table_name, column_name, options[:null], options[:default]) if options.key?(:null)
+            procs << Proc.new { change_column_comment(table_name, column_name, options[:comment]) } if options.key?(:comment)
+
+            [sqls, procs]
+          end
+
+
+          # Changes the default value of a table column.
+          def change_column_default_for_alter(table_name, column_name, default_or_changes) # :nodoc:
+            column = column_for(table_name, column_name)
+            return unless column
+
+            default = extract_new_default_value(default_or_changes)
+            alter_column_query = "ALTER COLUMN #{quote_column_name(column_name)} %s"
+            if default.nil?
+              # <tt>DEFAULT NULL</tt> results in the same behavior as <tt>DROP DEFAULT</tt>. However, PostgreSQL will
+              # cast the default to the columns type, which leaves us with a default like "default NULL::character varying".
+              alter_column_query % "DROP DEFAULT"
+            else
+              alter_column_query % "SET DEFAULT #{quote_default_expression(default, column)}"
+            end
+          end
+
+          def change_column_null_for_alter(table_name, column_name, null, default = nil) #:nodoc:
+            "ALTER #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL"
+          end
+
+          def add_index_opclass(quoted_columns, **options)
+            opclasses = options_for_index_columns(options[:opclass])
+            quoted_columns.each do |name, column|
+              column << " #{opclasses[name]}" if opclasses[name].present?
+            end
+          end
+
+          def add_options_for_index_columns(quoted_columns, **options)
+            quoted_columns = add_index_opclass(quoted_columns, options)
+            super
+          end
+
           def data_source_sql(name = nil, type: nil)
             scope = quoted_scope(name, type: type)
             scope[:type] ||= "'r','v','m'" # (r)elation/table, (v)iew, (m)aterialized view
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 4d37a292d6..23fc69d649 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -4,20 +4,20 @@
 gem "pg", "~> 0.18"
 require "pg"
 
-require_relative "abstract_adapter"
-require_relative "statement_pool"
-require_relative "postgresql/column"
-require_relative "postgresql/database_statements"
-require_relative "postgresql/explain_pretty_printer"
-require_relative "postgresql/oid"
-require_relative "postgresql/quoting"
-require_relative "postgresql/referential_integrity"
-require_relative "postgresql/schema_creation"
-require_relative "postgresql/schema_definitions"
-require_relative "postgresql/schema_dumper"
-require_relative "postgresql/schema_statements"
-require_relative "postgresql/type_metadata"
-require_relative "postgresql/utils"
+require "active_record/connection_adapters/abstract_adapter"
+require "active_record/connection_adapters/statement_pool"
+require "active_record/connection_adapters/postgresql/column"
+require "active_record/connection_adapters/postgresql/database_statements"
+require "active_record/connection_adapters/postgresql/explain_pretty_printer"
+require "active_record/connection_adapters/postgresql/oid"
+require "active_record/connection_adapters/postgresql/quoting"
+require "active_record/connection_adapters/postgresql/referential_integrity"
+require "active_record/connection_adapters/postgresql/schema_creation"
+require "active_record/connection_adapters/postgresql/schema_definitions"
+require "active_record/connection_adapters/postgresql/schema_dumper"
+require "active_record/connection_adapters/postgresql/schema_statements"
+require "active_record/connection_adapters/postgresql/type_metadata"
+require "active_record/connection_adapters/postgresql/utils"
 
 module ActiveRecord
   module ConnectionHandling # :nodoc:
@@ -142,6 +142,10 @@ module ActiveRecord
         true
       end
 
+      def supports_validate_constraints?
+        true
+      end
+
       def supports_views?
         true
       end
@@ -166,7 +170,7 @@ module ActiveRecord
         { concurrently: "CONCURRENTLY" }
       end
 
-      class StatementPool < ConnectionAdapters::StatementPool
+      class StatementPool < ConnectionAdapters::StatementPool # :nodoc:
         def initialize(connection, max)
           super(max)
           @connection = connection
@@ -182,7 +186,6 @@ module ActiveRecord
         end
 
         private
-
           def dealloc(key)
             @connection.query "DEALLOCATE #{key}" if connection_active?
           rescue PG::Error
@@ -273,6 +276,11 @@ module ActiveRecord
         end
       end
 
+      def discard! # :nodoc:
+        @connection.socket_io.reopen(IO::NULL)
+        @connection = nil
+      end
+
       def native_database_types #:nodoc:
         NATIVE_DATABASE_TYPES
       end
@@ -312,14 +320,14 @@ module ActiveRecord
 
       def get_advisory_lock(lock_id) # :nodoc:
         unless lock_id.is_a?(Integer) && lock_id.bit_length <= 63
-          raise(ArgumentError, "Postgres requires advisory lock ids to be a signed 64 bit integer")
+          raise(ArgumentError, "PostgreSQL requires advisory lock ids to be a signed 64 bit integer")
         end
         query_value("SELECT pg_try_advisory_lock(#{lock_id})")
       end
 
       def release_advisory_lock(lock_id) # :nodoc:
         unless lock_id.is_a?(Integer) && lock_id.bit_length <= 63
-          raise(ArgumentError, "Postgres requires advisory lock ids to be a signed 64 bit integer")
+          raise(ArgumentError, "PostgreSQL requires advisory lock ids to be a signed 64 bit integer")
         end
         query_value("SELECT pg_advisory_unlock(#{lock_id})")
       end
@@ -337,25 +345,20 @@ module ActiveRecord
       end
 
       def extension_enabled?(name)
-        if supports_extensions?
-          res = exec_query("SELECT EXISTS(SELECT * FROM pg_available_extensions WHERE name = '#{name}' AND installed_version IS NOT NULL) as enabled", "SCHEMA")
-          res.cast_values.first
-        end
+        res = exec_query("SELECT EXISTS(SELECT * FROM pg_available_extensions WHERE name = '#{name}' AND installed_version IS NOT NULL) as enabled", "SCHEMA")
+        res.cast_values.first
       end
 
       def extensions
-        if supports_extensions?
-          exec_query("SELECT extname FROM pg_extension", "SCHEMA").cast_values
-        else
-          super
-        end
+        exec_query("SELECT extname FROM pg_extension", "SCHEMA").cast_values
       end
 
       # Returns the configured supported identifier length supported by PostgreSQL
-      def table_alias_length
+      def max_identifier_length
         @max_identifier_length ||= query_value("SHOW max_identifier_length", "SCHEMA").to_i
       end
-      alias index_name_length table_alias_length
+      alias table_alias_length max_identifier_length
+      alias index_name_length max_identifier_length
 
       # Set the authorized user for this session
       def session_auth=(user)
@@ -387,7 +390,6 @@ module ActiveRecord
       end
 
       private
-
         # See https://www.postgresql.org/docs/current/static/errcodes-appendix.html
         VALUE_LIMIT_VIOLATION = "22001"
         NUMERIC_VALUE_OUT_OF_RANGE = "22003"
@@ -396,6 +398,8 @@ module ActiveRecord
         UNIQUE_VIOLATION      = "23505"
         SERIALIZATION_FAILURE = "40001"
         DEADLOCK_DETECTED     = "40P01"
+        LOCK_NOT_AVAILABLE    = "55P03"
+        QUERY_CANCELED        = "57014"
 
         def translate_exception(exception, message)
           return exception unless exception.respond_to?(:result)
@@ -415,6 +419,10 @@ module ActiveRecord
             SerializationFailure.new(message)
           when DEADLOCK_DETECTED
             Deadlocked.new(message)
+          when LOCK_NOT_AVAILABLE
+            LockWaitTimeout.new(message)
+          when QUERY_CANCELED
+            QueryCanceled.new(message)
           else
             super
           end
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/sqlite3/schema_definitions.rb
index 501f17dbad..c9855019c1 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3/schema_definitions.rb
@@ -3,27 +3,16 @@
 module ActiveRecord
   module ConnectionAdapters
     module SQLite3
-      module ColumnMethods
-        def primary_key(name, type = :primary_key, **options)
-          if %i(integer bigint).include?(type) && (options.delete(:auto_increment) == true || !options.key?(:default))
-            type = :primary_key
-          end
-
-          super
-        end
-      end
-
       class TableDefinition < ActiveRecord::ConnectionAdapters::TableDefinition
-        include ColumnMethods
-
         def references(*args, **options)
           super(*args, type: :integer, **options)
         end
         alias :belongs_to :references
-      end
 
-      class Table < ActiveRecord::ConnectionAdapters::Table
-        include ColumnMethods
+        private
+          def integer_like_primary_key_type(type, options)
+            :primary_key
+          end
       end
     end
   end
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb
index a512702b7b..58e5138e02 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb
@@ -5,13 +5,7 @@ module ActiveRecord
     module SQLite3
       module SchemaStatements # :nodoc:
         # Returns an array of indexes for the given table.
-        def indexes(table_name, name = nil)
-          if name
-            ActiveSupport::Deprecation.warn(<<-MSG.squish)
-              Passing name to #indexes is deprecated without replacement.
-            MSG
-          end
-
+        def indexes(table_name)
           exec_query("PRAGMA index_list(#{quote_table_name(table_name)})", "SCHEMA").map do |row|
             index_sql = query_value(<<-SQL, "SCHEMA")
               SELECT sql
@@ -29,20 +23,26 @@ module ActiveRecord
               col["name"]
             end
 
+            # Add info on sort order for columns (only desc order is explicitly specified, asc is
+            # the default)
+            orders = {}
+            if index_sql # index_sql can be null in case of primary key indexes
+              index_sql.scan(/"(\w+)" DESC/).flatten.each { |order_column|
+                orders[order_column] = :desc
+              }
+            end
+
             IndexDefinition.new(
               table_name,
               row["name"],
               row["unique"] != 0,
               columns,
-              where: where
+              where: where,
+              orders: orders
             )
           end
         end
 
-        def update_table_definition(table_name, base)
-          SQLite3::Table.new(table_name, base)
-        end
-
         def create_schema_dumper(options)
           SQLite3::SchemaDumper.create(self, options)
         end
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index 6fdd666486..c72db15ce3 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -1,13 +1,13 @@
 # frozen_string_literal: true
 
-require_relative "abstract_adapter"
-require_relative "statement_pool"
-require_relative "sqlite3/explain_pretty_printer"
-require_relative "sqlite3/quoting"
-require_relative "sqlite3/schema_creation"
-require_relative "sqlite3/schema_definitions"
-require_relative "sqlite3/schema_dumper"
-require_relative "sqlite3/schema_statements"
+require "active_record/connection_adapters/abstract_adapter"
+require "active_record/connection_adapters/statement_pool"
+require "active_record/connection_adapters/sqlite3/explain_pretty_printer"
+require "active_record/connection_adapters/sqlite3/quoting"
+require "active_record/connection_adapters/sqlite3/schema_creation"
+require "active_record/connection_adapters/sqlite3/schema_definitions"
+require "active_record/connection_adapters/sqlite3/schema_dumper"
+require "active_record/connection_adapters/sqlite3/schema_statements"
 
 gem "sqlite3", "~> 1.3.6"
 require "sqlite3"
@@ -70,7 +70,8 @@ module ActiveRecord
         time:         { name: "time" },
         date:         { name: "date" },
         binary:       { name: "blob" },
-        boolean:      { name: "boolean" }
+        boolean:      { name: "boolean" },
+        json:         { name: "json" },
       }
 
       ##
@@ -90,9 +91,8 @@ module ActiveRecord
       #   Rails.application.config.active_record.sqlite3.represent_boolean_as_integer = true
       class_attribute :represent_boolean_as_integer, default: false
 
-      class StatementPool < ConnectionAdapters::StatementPool
+      class StatementPool < ConnectionAdapters::StatementPool # :nodoc:
         private
-
           def dealloc(stmt)
             stmt[:stmt].close unless stmt[:stmt].closed?
           end
@@ -135,6 +135,10 @@ module ActiveRecord
         true
       end
 
+      def supports_json?
+        true
+      end
+
       def supports_multi_insert?
         sqlite_version >= "3.7.11"
       end
@@ -370,6 +374,10 @@ module ActiveRecord
       end
 
       private
+        def initialize_type_map(m = type_map)
+          super
+          register_class_with_limit m, %r(int)i, SQLite3Integer
+        end
 
         def table_structure(table_name)
           structure = exec_query("PRAGMA table_info(#{quote_table_name(table_name)})", "SCHEMA")
@@ -399,18 +407,21 @@ module ActiveRecord
           options[:id] = false
           create_table(to, options) do |definition|
             @definition = definition
-            @definition.primary_key(from_primary_key) if from_primary_key.present?
+            if from_primary_key.is_a?(Array)
+              @definition.primary_keys from_primary_key
+            end
             columns(from).each do |column|
               column_name = options[:rename] ?
                 (options[:rename][column.name] ||
                  options[:rename][column.name.to_sym] ||
                  column.name) : column.name
-              next if column_name == from_primary_key
 
               @definition.column(column_name, column.type,
                 limit: column.limit, default: column.default,
                 precision: column.precision, scale: column.scale,
-                null: column.null, collation: column.collation)
+                null: column.null, collation: column.collation,
+                primary_key: column_name == from_primary_key
+              )
             end
             yield @definition if block_given?
           end
@@ -423,6 +434,9 @@ module ActiveRecord
         def copy_table_indexes(from, to, rename = {})
           indexes(from).each do |index|
             name = index.name
+            # indexes sqlite creates for internal use start with `sqlite_` and
+            # don't need to be copied
+            next if name.starts_with?("sqlite_")
             if to == "a#{from}"
               name = "t#{name}"
             elsif from == "a#{to}"
@@ -525,6 +539,17 @@ module ActiveRecord
         def configure_connection
           execute("PRAGMA foreign_keys = ON", "SCHEMA")
         end
+
+        class SQLite3Integer < Type::Integer # :nodoc:
+          private
+            def _limit
+              # INTEGER storage class can be stored 8 bytes value.
+              # See https://www.sqlite.org/datatype3.html#storage_classes_and_datatypes
+              limit || 8
+            end
+        end
+
+        ActiveRecord::Type.register(:integer, SQLite3Integer, adapter: :sqlite3)
     end
     ActiveSupport.run_load_hooks(:active_record_sqlite3adapter, SQLite3Adapter)
   end
diff --git a/activerecord/lib/active_record/connection_handling.rb b/activerecord/lib/active_record/connection_handling.rb
index 9a47edfba4..88d28dc52a 100644
--- a/activerecord/lib/active_record/connection_handling.rb
+++ b/activerecord/lib/active_record/connection_handling.rb
@@ -140,6 +140,6 @@ module ActiveRecord
     end
 
     delegate :clear_active_connections!, :clear_reloadable_connections!,
-      :clear_all_connections!, to: :connection_handler
+      :clear_all_connections!, :flush_idle_connections!, to: :connection_handler
   end
 end
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 945c4eca78..481159e501 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -76,25 +76,13 @@ module ActiveRecord
       # scope being ignored is error-worthy, rather than a warning.
       mattr_accessor :error_on_ignored_order, instance_writer: false, default: false
 
-      def self.error_on_ignored_order_or_limit
-        ActiveSupport::Deprecation.warn(<<-MSG.squish)
-          The flag error_on_ignored_order_or_limit is deprecated. Limits are
-          now supported. Please use error_on_ignored_order instead.
-        MSG
-        error_on_ignored_order
-      end
-
-      def error_on_ignored_order_or_limit
-        self.class.error_on_ignored_order_or_limit
-      end
-
-      def self.error_on_ignored_order_or_limit=(value)
-        ActiveSupport::Deprecation.warn(<<-MSG.squish)
-          The flag error_on_ignored_order_or_limit is deprecated. Limits are
-          now supported. Please use error_on_ignored_order= instead.
-        MSG
-        self.error_on_ignored_order = value
-      end
+      # :singleton-method:
+      # Specify the behavior for unsafe raw query methods. Values are as follows
+      #   deprecated - Warnings are logged when unsafe raw SQL is passed to
+      #                query methods.
+      #   disabled   - Unsafe raw SQL passed to query methods results in
+      #                UnknownAttributeReference exception.
+      mattr_accessor :allow_unsafe_raw_sql, instance_writer: false, default: :deprecated
 
       ##
       # :singleton-method:
@@ -289,7 +277,8 @@ module ActiveRecord
           relation = Relation.create(self, arel_table, predicate_builder)
 
           if finder_needs_type_condition? && !ignore_default_scope?
-            relation.where(type_condition).create_with(inheritance_column.to_s => sti_name)
+            relation.where!(type_condition)
+            relation.create_with!(inheritance_column.to_s => sti_name)
           else
             relation
           end
diff --git a/activerecord/lib/active_record/counter_cache.rb b/activerecord/lib/active_record/counter_cache.rb
index 5005d58f1c..ee4f818cbf 100644
--- a/activerecord/lib/active_record/counter_cache.rb
+++ b/activerecord/lib/active_record/counter_cache.rb
@@ -53,7 +53,7 @@ module ActiveRecord
           unscoped.where(primary_key => object.id).update_all(updates)
         end
 
-        return true
+        true
       end
 
       # A generic "counter updater" implementation, intended primarily to be
diff --git a/activerecord/lib/active_record/enum.rb b/activerecord/lib/active_record/enum.rb
index f373b98035..1a3e6e4d09 100644
--- a/activerecord/lib/active_record/enum.rb
+++ b/activerecord/lib/active_record/enum.rb
@@ -221,6 +221,8 @@ module ActiveRecord
       def detect_enum_conflict!(enum_name, method_name, klass_method = false)
         if klass_method && dangerous_class_method?(method_name)
           raise_conflict_error(enum_name, method_name, type: "class")
+        elsif klass_method && method_defined_within?(method_name, Relation)
+          raise_conflict_error(enum_name, method_name, type: "class", source: Relation.name)
         elsif !klass_method && dangerous_attribute_method?(method_name)
           raise_conflict_error(enum_name, method_name)
         elsif !klass_method && method_defined_within?(method_name, _enum_methods_module, Module)
diff --git a/activerecord/lib/active_record/errors.rb b/activerecord/lib/active_record/errors.rb
index 9ef3316393..efcbd44776 100644
--- a/activerecord/lib/active_record/errors.rb
+++ b/activerecord/lib/active_record/errors.rb
@@ -335,8 +335,40 @@ module ActiveRecord
   class IrreversibleOrderError < ActiveRecordError
   end
 
-  # TransactionTimeout will be raised when lock wait timeout expires.
-  # Wait time value is set by innodb_lock_wait_timeout.
-  class TransactionTimeout < StatementInvalid
+  # LockWaitTimeout will be raised when lock wait timeout exceeded.
+  class LockWaitTimeout < StatementInvalid
+  end
+
+  # StatementTimeout will be raised when statement timeout exceeded.
+  class StatementTimeout < StatementInvalid
+  end
+
+  # QueryCanceled will be raised when canceling statement due to user request.
+  class QueryCanceled < StatementInvalid
+  end
+
+  # UnknownAttributeReference is raised when an unknown and potentially unsafe
+  # value is passed to a query method when allow_unsafe_raw_sql is set to
+  # :disabled. For example, passing a non column name value to a relation's
+  # #order method might cause this exception.
+  #
+  # When working around this exception, caution should be taken to avoid SQL
+  # injection vulnerabilities when passing user-provided values to query
+  # methods. Known-safe values can be passed to query methods by wrapping them
+  # in Arel.sql.
+  #
+  # For example, with allow_unsafe_raw_sql set to :disabled, the following
+  # code would raise this exception:
+  #
+  #   Post.order("length(title)").first
+  #
+  # The desired result can be accomplished by wrapping the known-safe string
+  # in Arel.sql:
+  #
+  #   Post.order(Arel.sql("length(title)")).first
+  #
+  # Again, such a workaround should *not* be used when passing user-provided
+  # values, such as request parameters or model attributes to query methods.
+  class UnknownAttributeReference < ActiveRecordError
   end
 end
diff --git a/activerecord/lib/active_record/explain.rb b/activerecord/lib/active_record/explain.rb
index 8bb54a24b7..7ccb938888 100644
--- a/activerecord/lib/active_record/explain.rb
+++ b/activerecord/lib/active_record/explain.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "explain_registry"
+require "active_record/explain_registry"
 
 module ActiveRecord
   module Explain
diff --git a/activerecord/lib/active_record/explain_subscriber.rb b/activerecord/lib/active_record/explain_subscriber.rb
index 9252fa3fed..a86217abc0 100644
--- a/activerecord/lib/active_record/explain_subscriber.rb
+++ b/activerecord/lib/active_record/explain_subscriber.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/notifications"
-require_relative "explain_registry"
+require "active_record/explain_registry"
 
 module ActiveRecord
   class ExplainSubscriber # :nodoc:
diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index 12169fffa9..86f13d75d5 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/activerecord/lib/active_record/fixtures.rb
@@ -6,8 +6,8 @@ require "zlib"
 require "set"
 require "active_support/dependencies"
 require "active_support/core_ext/digest/uuid"
-require_relative "fixture_set/file"
-require_relative "errors"
+require "active_record/fixture_set/file"
+require "active_record/errors"
 
 module ActiveRecord
   class FixtureClassNotFound < ActiveRecord::ActiveRecordError #:nodoc:
diff --git a/activerecord/lib/active_record/gem_version.rb b/activerecord/lib/active_record/gem_version.rb
index 7ccb57b305..7e47dac016 100644
--- a/activerecord/lib/active_record/gem_version.rb
+++ b/activerecord/lib/active_record/gem_version.rb
@@ -10,7 +10,7 @@ module ActiveRecord
     MAJOR = 5
     MINOR = 2
     TINY  = 0
-    PRE   = "alpha"
+    PRE   = "beta2"
 
     STRING = [MAJOR, MINOR, TINY, PRE].compact.join(".")
   end
diff --git a/activerecord/lib/active_record/internal_metadata.rb b/activerecord/lib/active_record/internal_metadata.rb
index 14795cc815..5a65edf27e 100644
--- a/activerecord/lib/active_record/internal_metadata.rb
+++ b/activerecord/lib/active_record/internal_metadata.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require_relative "scoping/default"
-require_relative "scoping/named"
+require "active_record/scoping/default"
+require "active_record/scoping/named"
 
 module ActiveRecord
   # This class is used to create a table that keeps track of values and keys such
diff --git a/activerecord/lib/active_record/legacy_yaml_adapter.rb b/activerecord/lib/active_record/legacy_yaml_adapter.rb
index 23644aab8f..ffa095dd94 100644
--- a/activerecord/lib/active_record/legacy_yaml_adapter.rb
+++ b/activerecord/lib/active_record/legacy_yaml_adapter.rb
@@ -8,7 +8,7 @@ module ActiveRecord
       case coder["active_record_yaml_version"]
       when 1, 2 then coder
       else
-        if coder["attributes"].is_a?(AttributeSet)
+        if coder["attributes"].is_a?(ActiveModel::AttributeSet)
           Rails420.convert(klass, coder)
         else
           Rails41.convert(klass, coder)
diff --git a/activerecord/lib/active_record/locking/pessimistic.rb b/activerecord/lib/active_record/locking/pessimistic.rb
index 72bccd4906..bb85c47e06 100644
--- a/activerecord/lib/active_record/locking/pessimistic.rb
+++ b/activerecord/lib/active_record/locking/pessimistic.rb
@@ -63,12 +63,13 @@ module ActiveRecord
       def lock!(lock = true)
         if persisted?
           if changed?
-            ActiveSupport::Deprecation.warn(<<-MSG.squish)
-              Locking a record with unpersisted changes is deprecated and will raise an
-              exception in Rails 5.2. Use `save` to persist the changes, or `reload` to
-              discard them explicitly.
+            raise(<<-MSG.squish)
+              Locking a record with unpersisted changes is not supported. Use
+              `save` to persist the changes, or `reload` to discard them
+              explicitly.
             MSG
           end
+
           reload(lock: lock)
         end
         self
diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index 52ca4671c2..5c10d4ff24 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -354,9 +354,9 @@ module ActiveRecord
   # to match the structure of your database.
   #
   # To roll the database back to a previous migration version, use
-  # <tt>rails db:migrate VERSION=X</tt> where <tt>X</tt> is the version to which
+  # <tt>rails db:rollback VERSION=X</tt> where <tt>X</tt> is the version to which
   # you wish to downgrade. Alternatively, you can also use the STEP option if you
-  # wish to rollback last few migrations. <tt>rails db:migrate STEP=2</tt> will rollback
+  # wish to rollback last few migrations. <tt>rails db:rollback STEP=2</tt> will rollback
   # the latest two migrations.
   #
   # If any of the migrations throw an <tt>ActiveRecord::IrreversibleMigration</tt> exception,
@@ -581,7 +581,8 @@ module ActiveRecord
       def load_schema_if_pending!
         if ActiveRecord::Migrator.needs_migration? || !ActiveRecord::Migrator.any_migrations?
           # Roundtrip to Rake to allow plugins to hook into database initialization.
-          FileUtils.cd Rails.root do
+          root = defined?(ENGINE_ROOT) ? ENGINE_ROOT : Rails.root
+          FileUtils.cd(root) do
             current_config = Base.connection_config
             Base.clear_all_connections!
             system("bin/rails db:test:prepare")
@@ -733,6 +734,24 @@ module ActiveRecord
       execute_block { yield helper }
     end
 
+    # Used to specify an operation that is only run when migrating up
+    # (for example, populating a new column with its initial values).
+    #
+    # In the following example, the new column +published+ will be given
+    # the value +true+ for all existing records.
+    #
+    #    class AddPublishedToPosts < ActiveRecord::Migration[5.2]
+    #      def change
+    #        add_column :posts, :published, :boolean, default: false
+    #        up_only do
+    #          execute "update posts set published = 'true'"
+    #        end
+    #      end
+    #    end
+    def up_only
+      execute_block { yield } unless reverting?
+    end
+
     # Runs the given migration classes.
     # Last argument can specify options:
     # - :direction (default is :up)
@@ -1026,12 +1045,7 @@ module ActiveRecord
         new(:up, migrations(migrations_paths), nil)
       end
 
-      def schema_migrations_table_name
-        SchemaMigration.table_name
-      end
-      deprecate :schema_migrations_table_name
-
-      def get_all_versions(connection = Base.connection)
+      def get_all_versions
         if SchemaMigration.table_exists?
           SchemaMigration.all_versions.map(&:to_i)
         else
@@ -1039,12 +1053,13 @@ module ActiveRecord
         end
       end
 
-      def current_version(connection = Base.connection)
-        get_all_versions(connection).max || 0
+      def current_version(connection = nil)
+        get_all_versions.max || 0
+      rescue ActiveRecord::NoDatabaseError
       end
 
-      def needs_migration?(connection = Base.connection)
-        (migrations(migrations_paths).collect(&:version) - get_all_versions(connection)).size > 0
+      def needs_migration?(connection = nil)
+        (migrations(migrations_paths).collect(&:version) - get_all_versions).size > 0
       end
 
       def any_migrations?
@@ -1229,7 +1244,7 @@ module ActiveRecord
 
       # Return true if a valid version is not provided.
       def invalid_target?
-        !target && @target_version && @target_version > 0
+        @target_version && @target_version != 0 && !target
       end
 
       def execute_migration_in_transaction(migration, direction)
diff --git a/activerecord/lib/active_record/migration/command_recorder.rb b/activerecord/lib/active_record/migration/command_recorder.rb
index a3a5e0fa16..81ef4828f8 100644
--- a/activerecord/lib/active_record/migration/command_recorder.rb
+++ b/activerecord/lib/active_record/migration/command_recorder.rb
@@ -110,7 +110,7 @@ module ActiveRecord
 
       private
 
-        module StraightReversions
+        module StraightReversions # :nodoc:
           private
             { transaction:       :transaction,
               execute_block:     :execute_block,
@@ -161,8 +161,8 @@ module ActiveRecord
           table, columns, options = *args
           options ||= {}
 
-          index_name = options[:name]
-          options_hash = index_name ? { name: index_name } : { column: columns }
+          options_hash = options.slice(:name, :algorithm)
+          options_hash[:column] = columns if !options_hash[:name]
 
           [:remove_index, [table, options_hash]]
         end
diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
index 784292f3f9..bd8c054c28 100644
--- a/activerecord/lib/active_record/migration/compatibility.rb
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -16,10 +16,27 @@ module ActiveRecord
       V5_2 = Current
 
       class V5_1 < V5_2
+        def change_column(table_name, column_name, type, options = {})
+          if adapter_name == "PostgreSQL"
+            clear_cache!
+            sql = connection.send(:change_column_sql, table_name, column_name, type, options)
+            execute "ALTER TABLE #{quote_table_name(table_name)} #{sql}"
+            change_column_default(table_name, column_name, options[:default]) if options.key?(:default)
+            change_column_null(table_name, column_name, options[:null], options[:default]) if options.key?(:null)
+            change_column_comment(table_name, column_name, options[:comment]) if options.key?(:comment)
+          else
+            super
+          end
+        end
       end
 
       class V5_0 < V5_1
         module TableDefinition
+          def primary_key(name, type = :primary_key, **options)
+            type = :integer if type == :primary_key
+            super
+          end
+
           def references(*args, **options)
             super(*args, type: :integer, **options)
           end
@@ -47,11 +64,8 @@ module ActiveRecord
           end
 
           if block_given?
-            super(table_name, options) do |t|
-              class << t
-                prepend TableDefinition
-              end
-              yield t
+            super do |t|
+              yield compatible_table_definition(t)
             end
           else
             super
@@ -60,21 +74,46 @@ module ActiveRecord
 
         def change_table(table_name, options = {})
           if block_given?
-            super(table_name, options) do |t|
-              class << t
-                prepend TableDefinition
-              end
-              yield t
+            super do |t|
+              yield compatible_table_definition(t)
             end
           else
             super
           end
         end
 
+        def create_join_table(table_1, table_2, column_options: {}, **options)
+          column_options.reverse_merge!(type: :integer)
+
+          if block_given?
+            super do |t|
+              yield compatible_table_definition(t)
+            end
+          else
+            super
+          end
+        end
+
+        def add_column(table_name, column_name, type, options = {})
+          if type == :primary_key
+            type = :integer
+            options[:primary_key] = true
+          end
+          super
+        end
+
         def add_reference(table_name, ref_name, **options)
           super(table_name, ref_name, type: :integer, **options)
         end
         alias :add_belongs_to :add_reference
+
+        private
+          def compatible_table_definition(t)
+            class << t
+              prepend TableDefinition
+            end
+            t
+          end
       end
 
       class V4_2 < V5_0
@@ -93,11 +132,8 @@ module ActiveRecord
 
         def create_table(table_name, options = {})
           if block_given?
-            super(table_name, options) do |t|
-              class << t
-                prepend TableDefinition
-              end
-              yield t
+            super do |t|
+              yield compatible_table_definition(t)
             end
           else
             super
@@ -106,11 +142,8 @@ module ActiveRecord
 
         def change_table(table_name, options = {})
           if block_given?
-            super(table_name, options) do |t|
-              class << t
-                prepend TableDefinition
-              end
-              yield t
+            super do |t|
+              yield compatible_table_definition(t)
             end
           else
             super
@@ -146,6 +179,12 @@ module ActiveRecord
         end
 
         private
+          def compatible_table_definition(t)
+            class << t
+              prepend TableDefinition
+            end
+            super
+          end
 
           def index_name_for_remove(table_name, options = {})
             index_name = index_name(table_name, options)
diff --git a/activerecord/lib/active_record/model_schema.rb b/activerecord/lib/active_record/model_schema.rb
index 34c0ef4e75..1941d3d5ea 100644
--- a/activerecord/lib/active_record/model_schema.rb
+++ b/activerecord/lib/active_record/model_schema.rb
@@ -115,20 +115,6 @@ module ActiveRecord
     # If true, the default table name for a Product class will be "products". If false, it would just be "product".
     # See table_name for the full rules on table/class naming. This is true, by default.
 
-    ##
-    # :singleton-method: ignored_columns
-    # :call-seq: ignored_columns
-    #
-    # The list of columns names the model should ignore. Ignored columns won't have attribute
-    # accessors defined, and won't be referenced in SQL queries.
-
-    ##
-    # :singleton-method: ignored_columns=
-    # :call-seq: ignored_columns=(columns)
-    #
-    # Sets the columns names the model should ignore. Ignored columns won't have attribute
-    # accessors defined, and won't be referenced in SQL queries.
-
     included do
       mattr_accessor :primary_key_prefix_type, instance_writer: false
 
@@ -138,9 +124,9 @@ module ActiveRecord
       class_attribute :internal_metadata_table_name, instance_accessor: false, default: "ar_internal_metadata"
       class_attribute :protected_environments, instance_accessor: false, default: [ "production" ]
       class_attribute :pluralize_table_names, instance_writer: false, default: true
-      class_attribute :ignored_columns, instance_accessor: false, default: [].freeze
 
       self.inheritance_column = "type"
+      self.ignored_columns = [].freeze
 
       delegate :type_for_attribute, to: :class
 
@@ -271,6 +257,22 @@ module ActiveRecord
         @explicit_inheritance_column = true
       end
 
+      # The list of columns names the model should ignore. Ignored columns won't have attribute
+      # accessors defined, and won't be referenced in SQL queries.
+      def ignored_columns
+        if defined?(@ignored_columns)
+          @ignored_columns
+        else
+          superclass.ignored_columns
+        end
+      end
+
+      # Sets the columns names the model should ignore. Ignored columns won't have attribute
+      # accessors defined, and won't be referenced in SQL queries.
+      def ignored_columns=(columns)
+        @ignored_columns = columns.map(&:to_s)
+      end
+
       def sequence_name
         if base_class == self
           @sequence_name ||= reset_sequence_name
@@ -321,11 +323,11 @@ module ActiveRecord
       end
 
       def attributes_builder # :nodoc:
-        @attributes_builder ||= AttributeSet::Builder.new(attribute_types, primary_key) do |name|
-          unless columns_hash.key?(name)
-            _default_attributes[name].dup
-          end
+        unless defined?(@attributes_builder) && @attributes_builder
+          defaults = _default_attributes.except(*(column_names - [primary_key]))
+          @attributes_builder = ActiveModel::AttributeSet::Builder.new(attribute_types, defaults)
         end
+        @attributes_builder
       end
 
       def columns_hash # :nodoc:
@@ -344,7 +346,7 @@ module ActiveRecord
       end
 
       def yaml_encoder # :nodoc:
-        @yaml_encoder ||= AttributeSet::YAMLEncoder.new(attribute_types)
+        @yaml_encoder ||= ActiveModel::AttributeSet::YAMLEncoder.new(attribute_types)
       end
 
       # Returns the type of the attribute with the given name, after applying
@@ -374,7 +376,7 @@ module ActiveRecord
       end
 
       def _default_attributes # :nodoc:
-        @default_attributes ||= AttributeSet.new({})
+        @default_attributes ||= ActiveModel::AttributeSet.new({})
       end
 
       # Returns an array of column names as strings.
diff --git a/activerecord/lib/active_record/nested_attributes.rb b/activerecord/lib/active_record/nested_attributes.rb
index 435c81c153..fa20bce3a9 100644
--- a/activerecord/lib/active_record/nested_attributes.rb
+++ b/activerecord/lib/active_record/nested_attributes.rb
@@ -63,6 +63,18 @@ module ActiveRecord
     #   member.update params[:member]
     #   member.avatar.icon # => 'sad'
     #
+    # If you want to update the current avatar without providing the id, you must add <tt>:update_only</tt> option.
+    #
+    #   class Member < ActiveRecord::Base
+    #     has_one :avatar
+    #     accepts_nested_attributes_for :avatar, update_only: true
+    #   end
+    #
+    #   params = { member: { avatar_attributes: { icon: 'sad' } } }
+    #   member.update params[:member]
+    #   member.avatar.id # => 2
+    #   member.avatar.icon # => 'sad'
+    #
     # By default you will only be able to set and update attributes on the
     # associated model. If you want to destroy the associated model through the
     # attributes hash, you have to enable it first using the
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index fbbf9082cc..a13b0d0181 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -71,6 +71,132 @@ module ActiveRecord
         klass.allocate.init_with("attributes" => attributes, "new_record" => false, &block)
       end
 
+      # Updates an object (or multiple objects) and saves it to the database, if validations pass.
+      # The resulting object is returned whether the object was saved successfully to the database or not.
+      #
+      # ==== Parameters
+      #
+      # * +id+ - This should be the id or an array of ids to be updated.
+      # * +attributes+ - This should be a hash of attributes or an array of hashes.
+      #
+      # ==== Examples
+      #
+      #   # Updates one record
+      #   Person.update(15, user_name: "Samuel", group: "expert")
+      #
+      #   # Updates multiple records
+      #   people = { 1 => { "first_name" => "David" }, 2 => { "first_name" => "Jeremy" } }
+      #   Person.update(people.keys, people.values)
+      #
+      #   # Updates multiple records from the result of a relation
+      #   people = Person.where(group: "expert")
+      #   people.update(group: "masters")
+      #
+      # Note: Updating a large number of records will run an UPDATE
+      # query for each record, which may cause a performance issue.
+      # When running callbacks is not needed for each record update,
+      # it is preferred to use {update_all}[rdoc-ref:Relation#update_all]
+      # for updating all records in a single query.
+      def update(id = :all, attributes)
+        if id.is_a?(Array)
+          id.map { |one_id| find(one_id) }.each_with_index { |object, idx|
+            object.update(attributes[idx])
+          }
+        elsif id == :all
+          all.each { |record| record.update(attributes) }
+        else
+          if ActiveRecord::Base === id
+            raise ArgumentError,
+              "You are passing an instance of ActiveRecord::Base to `update`. " \
+              "Please pass the id of the object by calling `.id`."
+          end
+          object = find(id)
+          object.update(attributes)
+          object
+        end
+      end
+
+      # Destroy an object (or multiple objects) that has the given id. The object is instantiated first,
+      # therefore all callbacks and filters are fired off before the object is deleted. This method is
+      # less efficient than #delete but allows cleanup methods and other actions to be run.
+      #
+      # This essentially finds the object (or multiple objects) with the given id, creates a new object
+      # from the attributes, and then calls destroy on it.
+      #
+      # ==== Parameters
+      #
+      # * +id+ - This should be the id or an array of ids to be destroyed.
+      #
+      # ==== Examples
+      #
+      #   # Destroy a single object
+      #   Todo.destroy(1)
+      #
+      #   # Destroy multiple objects
+      #   todos = [1,2,3]
+      #   Todo.destroy(todos)
+      def destroy(id)
+        if id.is_a?(Array)
+          find(id).each(&:destroy)
+        else
+          find(id).destroy
+        end
+      end
+
+      # Deletes the row with a primary key matching the +id+ argument, using a
+      # SQL +DELETE+ statement, and returns the number of rows deleted. Active
+      # Record objects are not instantiated, so the object's callbacks are not
+      # executed, including any <tt>:dependent</tt> association options.
+      #
+      # You can delete multiple rows at once by passing an Array of <tt>id</tt>s.
+      #
+      # Note: Although it is often much faster than the alternative, #destroy,
+      # skipping callbacks might bypass business logic in your application
+      # that ensures referential integrity or performs other essential jobs.
+      #
+      # ==== Examples
+      #
+      #   # Delete a single row
+      #   Todo.delete(1)
+      #
+      #   # Delete multiple rows
+      #   Todo.delete([2,3,4])
+      def delete(id_or_array)
+        where(primary_key => id_or_array).delete_all
+      end
+
+      def _insert_record(values) # :nodoc:
+        primary_key_value = nil
+
+        if primary_key && Hash === values
+          arel_primary_key = arel_attribute(primary_key)
+          primary_key_value = values[arel_primary_key]
+
+          if !primary_key_value && prefetch_primary_key?
+            primary_key_value = next_sequence_value
+            values[arel_primary_key] = primary_key_value
+          end
+        end
+
+        if values.empty?
+          im = arel_table.compile_insert(connection.empty_insert_statement_value)
+          im.into arel_table
+        else
+          im = arel_table.compile_insert(_substitute_values(values))
+        end
+
+        connection.insert(im, "#{self} Create", primary_key || false, primary_key_value)
+      end
+
+      def _update_record(values, id, id_was) # :nodoc:
+        bind = predicate_builder.build_bind_attribute(primary_key, id_was || id)
+        um = arel_table.where(
+          arel_attribute(primary_key).eq(bind)
+        ).compile_update(_substitute_values(values), primary_key)
+
+        connection.update(um, "#{self} Update")
+      end
+
       private
         # Called by +instantiate+ to decide which class to use for a new
         # record instance.
@@ -80,6 +206,13 @@ module ActiveRecord
         def discriminate_class_for_record(record)
           self
         end
+
+        def _substitute_values(values)
+          values.map do |attr, value|
+            bind = predicate_builder.build_bind_attribute(attr.name, value)
+            [attr, bind]
+          end
+        end
     end
 
     # Returns true if this object hasn't been saved yet -- that is, a record
@@ -228,7 +361,7 @@ module ActiveRecord
     def becomes(klass)
       became = klass.new
       became.instance_variable_set("@attributes", @attributes)
-      became.instance_variable_set("@mutation_tracker", @mutation_tracker) if defined?(@mutation_tracker)
+      became.instance_variable_set("@mutations_from_database", @mutations_from_database) if defined?(@mutations_from_database)
       became.instance_variable_set("@changed_attributes", attributes_changed_by_setter)
       became.instance_variable_set("@new_record", new_record?)
       became.instance_variable_set("@destroyed", destroyed?)
@@ -577,7 +710,7 @@ module ActiveRecord
         rows_affected = 0
         @_trigger_update_callback = true
       else
-        rows_affected = self.class.unscoped._update_record attributes_values, id, id_in_database
+        rows_affected = self.class._update_record(attributes_values, id, id_in_database)
         @_trigger_update_callback = rows_affected > 0
       end
 
@@ -591,7 +724,7 @@ module ActiveRecord
     def _create_record(attribute_names = self.attribute_names)
       attributes_values = arel_attributes_with_values_for_create(attribute_names)
 
-      new_id = self.class.unscoped.insert attributes_values
+      new_id = self.class._insert_record(attributes_values)
       self.id ||= new_id if self.class.primary_key
 
       @new_record = false
diff --git a/activerecord/lib/active_record/query_cache.rb b/activerecord/lib/active_record/query_cache.rb
index 3d5babb8b7..8e23128333 100644
--- a/activerecord/lib/active_record/query_cache.rb
+++ b/activerecord/lib/active_record/query_cache.rb
@@ -26,16 +26,19 @@ module ActiveRecord
     end
 
     def self.run
-      caching_pool = ActiveRecord::Base.connection_pool
-      caching_was_enabled = caching_pool.query_cache_enabled
+      ActiveRecord::Base.connection_handler.connection_pool_list.map do |pool|
+        caching_was_enabled = pool.query_cache_enabled
 
-      caching_pool.enable_query_cache!
+        pool.enable_query_cache!
 
-      [caching_pool, caching_was_enabled]
+        [pool, caching_was_enabled]
+      end
     end
 
-    def self.complete((caching_pool, caching_was_enabled))
-      caching_pool.disable_query_cache! unless caching_was_enabled
+    def self.complete(caching_pools)
+      caching_pools.each do |pool, caching_was_enabled|
+        pool.disable_query_cache! unless caching_was_enabled
+      end
 
       ActiveRecord::Base.connection_handler.connection_pool_list.each do |pool|
         pool.release_connection if pool.active_connection? && !pool.connection.transaction_open?
diff --git a/activerecord/lib/active_record/querying.rb b/activerecord/lib/active_record/querying.rb
index f780538319..3996d5661f 100644
--- a/activerecord/lib/active_record/querying.rb
+++ b/activerecord/lib/active_record/querying.rb
@@ -7,7 +7,7 @@ module ActiveRecord
     delegate :first_or_create, :first_or_create!, :first_or_initialize, to: :all
     delegate :find_or_create_by, :find_or_create_by!, :find_or_initialize_by, to: :all
     delegate :find_by, :find_by!, to: :all
-    delegate :destroy, :destroy_all, :delete, :delete_all, :update, :update_all, to: :all
+    delegate :destroy_all, :delete_all, :update_all, to: :all
     delegate :find_each, :find_in_batches, :in_batches, to: :all
     delegate :select, :group, :order, :except, :reorder, :limit, :offset, :joins, :left_joins, :left_outer_joins, :or,
              :where, :rewhere, :preload, :eager_load, :includes, :from, :lock, :readonly, :extending,
diff --git a/activerecord/lib/active_record/railtie.rb b/activerecord/lib/active_record/railtie.rb
index ead42d64ec..4538ed6a5f 100644
--- a/activerecord/lib/active_record/railtie.rb
+++ b/activerecord/lib/active_record/railtie.rb
@@ -53,8 +53,8 @@ module ActiveRecord
     # to avoid cross references when loading a constant for the
     # first time. Also, make it output to STDERR.
     console do |app|
-      require_relative "railties/console_sandbox" if app.sandbox?
-      require_relative "base"
+      require "active_record/railties/console_sandbox" if app.sandbox?
+      require "active_record/base"
       unless ActiveSupport::Logger.logger_outputs_to?(Rails.logger, STDERR, STDOUT)
         console = ActiveSupport::Logger.new(STDERR)
         Rails.logger.extend ActiveSupport::Logger.broadcast console
@@ -62,7 +62,7 @@ module ActiveRecord
     end
 
     runner do
-      require_relative "base"
+      require "active_record/base"
     end
 
     initializer "active_record.initialize_timezone" do
@@ -90,12 +90,15 @@ module ActiveRecord
             filename = File.join(app.config.paths["db"].first, "schema_cache.yml")
 
             if File.file?(filename)
+              current_version = ActiveRecord::Migrator.current_version
+              next if current_version.nil?
+
               cache = YAML.load(File.read(filename))
-              if cache.version == ActiveRecord::Migrator.current_version
+              if cache.version == current_version
                 connection.schema_cache = cache
                 connection_pool.schema_cache = cache.dup
               else
-                warn "Ignoring db/schema_cache.yml because it has expired. The current schema version is #{ActiveRecord::Migrator.current_version}, but the one in the cache is #{cache.version}."
+                warn "Ignoring db/schema_cache.yml because it has expired. The current schema version is #{current_version}, but the one in the cache is #{cache.version}."
               end
             end
           end
@@ -106,7 +109,7 @@ module ActiveRecord
     initializer "active_record.warn_on_records_fetched_greater_than" do
       if config.active_record.warn_on_records_fetched_greater_than
         ActiveSupport.on_load(:active_record) do
-          require_relative "relation/record_fetch_warning"
+          require "active_record/relation/record_fetch_warning"
         end
       end
     end
@@ -146,7 +149,7 @@ end_warning
 
     # Expose database runtime to controller for logging.
     initializer "active_record.log_runtime" do
-      require_relative "railties/controller_runtime"
+      require "active_record/railties/controller_runtime"
       ActiveSupport.on_load(:action_controller) do
         include ActiveRecord::Railties::ControllerRuntime
       end
@@ -177,7 +180,16 @@ end_warning
     initializer "active_record.clear_active_connections" do
       config.after_initialize do
         ActiveSupport.on_load(:active_record) do
+          # Ideally the application doesn't connect to the database during boot,
+          # but sometimes it does. In case it did, we want to empty out the
+          # connection pools so that a non-database-using process (e.g. a master
+          # process in a forking server model) doesn't retain a needless
+          # connection. If it was needed, the incremental cost of reestablishing
+          # this connection is trivial: the rest of the pool would need to be
+          # populated anyway.
+
           clear_active_connections!
+          flush_idle_connections!
         end
       end
     end
diff --git a/activerecord/lib/active_record/railties/controller_runtime.rb b/activerecord/lib/active_record/railties/controller_runtime.rb
index 3cf66980a5..2ae733f657 100644
--- a/activerecord/lib/active_record/railties/controller_runtime.rb
+++ b/activerecord/lib/active_record/railties/controller_runtime.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/module/attr_internal"
-require_relative "../log_subscriber"
+require "active_record/log_subscriber"
 
 module ActiveRecord
   module Railties # :nodoc:
diff --git a/activerecord/lib/active_record/railties/databases.rake b/activerecord/lib/active_record/railties/databases.rake
index 691b3612d8..fce3e1c5cf 100644
--- a/activerecord/lib/active_record/railties/databases.rake
+++ b/activerecord/lib/active_record/railties/databases.rake
@@ -4,16 +4,16 @@ require "active_record"
 
 db_namespace = namespace :db do
   desc "Set the environment value for the database"
-  task "environment:set" => [:environment, :load_config] do
+  task "environment:set" => :load_config do
     ActiveRecord::InternalMetadata.create_table
     ActiveRecord::InternalMetadata[:environment] = ActiveRecord::Migrator.current_environment
   end
 
-  task check_protected_environments: [:environment, :load_config] do
+  task check_protected_environments: :load_config do
     ActiveRecord::Tasks::DatabaseTasks.check_protected_environments!
   end
 
-  task :load_config do
+  task load_config: :environment do
     ActiveRecord::Base.configurations       = ActiveRecord::Tasks::DatabaseTasks.database_configuration || {}
     ActiveRecord::Migrator.migrations_paths = ActiveRecord::Tasks::DatabaseTasks.migrations_paths
   end
@@ -56,7 +56,7 @@ db_namespace = namespace :db do
   end
 
   desc "Migrate the database (options: VERSION=x, VERBOSE=false, SCOPE=blog)."
-  task migrate: [:environment, :load_config] do
+  task migrate: :load_config do
     ActiveRecord::Tasks::DatabaseTasks.migrate
     db_namespace["_dump"].invoke
   end
@@ -78,7 +78,7 @@ db_namespace = namespace :db do
 
   namespace :migrate do
     # desc  'Rollbacks the database one migration and re migrate up (options: STEP=x, VERSION=x).'
-    task redo: [:environment, :load_config] do
+    task redo: :load_config do
       raise "Empty VERSION provided" if ENV["VERSION"] && ENV["VERSION"].empty?
 
       if ENV["VERSION"]
@@ -94,24 +94,35 @@ db_namespace = namespace :db do
     task reset: ["db:drop", "db:create", "db:migrate"]
 
     # desc 'Runs the "up" for a given migration VERSION.'
-    task up: [:environment, :load_config] do
+    task up: :load_config do
       raise "VERSION is required" if !ENV["VERSION"] || ENV["VERSION"].empty?
 
-      version = ENV["VERSION"] ? ENV["VERSION"].to_i : nil
-      ActiveRecord::Migrator.run(:up, ActiveRecord::Tasks::DatabaseTasks.migrations_paths, version)
+      ActiveRecord::Tasks::DatabaseTasks.check_target_version
+
+      ActiveRecord::Migrator.run(
+        :up,
+        ActiveRecord::Tasks::DatabaseTasks.migrations_paths,
+        ActiveRecord::Tasks::DatabaseTasks.target_version
+      )
       db_namespace["_dump"].invoke
     end
 
     # desc 'Runs the "down" for a given migration VERSION.'
-    task down: [:environment, :load_config] do
+    task down: :load_config do
       raise "VERSION is required - To go down one migration, use db:rollback" if !ENV["VERSION"] || ENV["VERSION"].empty?
-      version = ENV["VERSION"] ? ENV["VERSION"].to_i : nil
-      ActiveRecord::Migrator.run(:down, ActiveRecord::Tasks::DatabaseTasks.migrations_paths, version)
+
+      ActiveRecord::Tasks::DatabaseTasks.check_target_version
+
+      ActiveRecord::Migrator.run(
+        :down,
+        ActiveRecord::Tasks::DatabaseTasks.migrations_paths,
+        ActiveRecord::Tasks::DatabaseTasks.target_version
+      )
       db_namespace["_dump"].invoke
     end
 
     desc "Display status of migrations"
-    task status: [:environment, :load_config] do
+    task status: :load_config do
       unless ActiveRecord::SchemaMigration.table_exists?
         abort "Schema migrations table does not exist yet."
       end
@@ -129,14 +140,14 @@ db_namespace = namespace :db do
   end
 
   desc "Rolls the schema back to the previous version (specify steps w/ STEP=n)."
-  task rollback: [:environment, :load_config] do
+  task rollback: :load_config do
     step = ENV["STEP"] ? ENV["STEP"].to_i : 1
     ActiveRecord::Migrator.rollback(ActiveRecord::Tasks::DatabaseTasks.migrations_paths, step)
     db_namespace["_dump"].invoke
   end
 
   # desc 'Pushes the schema to the next version (specify steps w/ STEP=n).'
-  task forward: [:environment, :load_config] do
+  task forward: :load_config do
     step = ENV["STEP"] ? ENV["STEP"].to_i : 1
     ActiveRecord::Migrator.forward(ActiveRecord::Tasks::DatabaseTasks.migrations_paths, step)
     db_namespace["_dump"].invoke
@@ -146,12 +157,12 @@ db_namespace = namespace :db do
   task reset: [ "db:drop", "db:setup" ]
 
   # desc "Retrieves the charset for the current environment's database"
-  task charset: [:environment, :load_config] do
+  task charset: :load_config do
     puts ActiveRecord::Tasks::DatabaseTasks.charset_current
   end
 
   # desc "Retrieves the collation for the current environment's database"
-  task collation: [:environment, :load_config] do
+  task collation: :load_config do
     begin
       puts ActiveRecord::Tasks::DatabaseTasks.collation_current
     rescue NoMethodError
@@ -160,12 +171,12 @@ db_namespace = namespace :db do
   end
 
   desc "Retrieves the current schema version number"
-  task version: [:environment, :load_config] do
+  task version: :load_config do
     puts "Current version: #{ActiveRecord::Migrator.current_version}"
   end
 
   # desc "Raises an error if there are pending migrations"
-  task abort_if_pending_migrations: [:environment, :load_config] do
+  task abort_if_pending_migrations: :load_config do
     pending_migrations = ActiveRecord::Migrator.open(ActiveRecord::Tasks::DatabaseTasks.migrations_paths).pending_migrations
 
     if pending_migrations.any?
@@ -188,8 +199,8 @@ db_namespace = namespace :db do
 
   namespace :fixtures do
     desc "Loads fixtures into the current environment's database. Load specific fixtures using FIXTURES=x,y. Load from subdirectory in test/fixtures using FIXTURES_DIR=z. Specify an alternative path (eg. spec/fixtures) using FIXTURES_PATH=spec/fixtures."
-    task load: [:environment, :load_config] do
-      require_relative "../fixtures"
+    task load: :load_config do
+      require "active_record/fixtures"
 
       base_dir = ActiveRecord::Tasks::DatabaseTasks.fixtures_path
 
@@ -210,8 +221,8 @@ db_namespace = namespace :db do
     end
 
     # desc "Search for a fixture given a LABEL or ID. Specify an alternative path (eg. spec/fixtures) using FIXTURES_PATH=spec/fixtures."
-    task identify: [:environment, :load_config] do
-      require_relative "../fixtures"
+    task identify: :load_config do
+      require "active_record/fixtures"
 
       label, id = ENV["LABEL"], ENV["ID"]
       raise "LABEL or ID required" if label.blank? && id.blank?
@@ -236,8 +247,8 @@ db_namespace = namespace :db do
 
   namespace :schema do
     desc "Creates a db/schema.rb file that is portable against any DB supported by Active Record"
-    task dump: [:environment, :load_config] do
-      require_relative "../schema_dumper"
+    task dump: :load_config do
+      require "active_record/schema_dumper"
       filename = ENV["SCHEMA"] || File.join(ActiveRecord::Tasks::DatabaseTasks.db_dir, "schema.rb")
       File.open(filename, "w:utf-8") do |file|
         ActiveRecord::SchemaDumper.dump(ActiveRecord::Base.connection, file)
@@ -246,7 +257,7 @@ db_namespace = namespace :db do
     end
 
     desc "Loads a schema.rb file into the database"
-    task load: [:environment, :load_config, :check_protected_environments] do
+    task load: [:load_config, :check_protected_environments] do
       ActiveRecord::Tasks::DatabaseTasks.load_schema_current(:ruby, ENV["SCHEMA"])
     end
 
@@ -256,14 +267,14 @@ db_namespace = namespace :db do
 
     namespace :cache do
       desc "Creates a db/schema_cache.yml file."
-      task dump: [:environment, :load_config] do
+      task dump: :load_config do
         conn = ActiveRecord::Base.connection
         filename = File.join(ActiveRecord::Tasks::DatabaseTasks.db_dir, "schema_cache.yml")
         ActiveRecord::Tasks::DatabaseTasks.dump_schema_cache(conn, filename)
       end
 
       desc "Clears a db/schema_cache.yml file."
-      task clear: [:environment, :load_config] do
+      task clear: :load_config do
         filename = File.join(ActiveRecord::Tasks::DatabaseTasks.db_dir, "schema_cache.yml")
         rm_f filename, verbose: false
       end
@@ -273,7 +284,7 @@ db_namespace = namespace :db do
 
   namespace :structure do
     desc "Dumps the database structure to db/structure.sql. Specify another file with SCHEMA=db/my_structure.sql"
-    task dump: [:environment, :load_config] do
+    task dump: :load_config do
       filename = ENV["SCHEMA"] || File.join(ActiveRecord::Tasks::DatabaseTasks.db_dir, "structure.sql")
       current_config = ActiveRecord::Tasks::DatabaseTasks.current_config
       ActiveRecord::Tasks::DatabaseTasks.structure_dump(current_config, filename)
@@ -288,7 +299,7 @@ db_namespace = namespace :db do
     end
 
     desc "Recreates the databases from the structure.sql file"
-    task load: [:environment, :load_config, :check_protected_environments] do
+    task load: [:load_config, :check_protected_environments] do
       ActiveRecord::Tasks::DatabaseTasks.load_schema_current(:sql, ENV["SCHEMA"])
     end
 
@@ -313,7 +324,7 @@ db_namespace = namespace :db do
       begin
         should_reconnect = ActiveRecord::Base.connection_pool.active_connection?
         ActiveRecord::Schema.verbose = false
-        ActiveRecord::Tasks::DatabaseTasks.load_schema ActiveRecord::Base.configurations["test"], :ruby, ENV["SCHEMA"]
+        ActiveRecord::Tasks::DatabaseTasks.load_schema ActiveRecord::Base.configurations["test"], :ruby, ENV["SCHEMA"], "test"
       ensure
         if should_reconnect
           ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations[ActiveRecord::Tasks::DatabaseTasks.env])
@@ -323,16 +334,16 @@ db_namespace = namespace :db do
 
     # desc "Recreate the test database from an existent structure.sql file"
     task load_structure: %w(db:test:purge) do
-      ActiveRecord::Tasks::DatabaseTasks.load_schema ActiveRecord::Base.configurations["test"], :sql, ENV["SCHEMA"]
+      ActiveRecord::Tasks::DatabaseTasks.load_schema ActiveRecord::Base.configurations["test"], :sql, ENV["SCHEMA"], "test"
     end
 
     # desc "Empty the test database"
-    task purge: %w(environment load_config check_protected_environments) do
+    task purge: %w(load_config check_protected_environments) do
       ActiveRecord::Tasks::DatabaseTasks.purge ActiveRecord::Base.configurations["test"]
     end
 
     # desc 'Load the test schema'
-    task prepare: %w(environment load_config) do
+    task prepare: :load_config do
       unless ActiveRecord::Base.configurations.blank?
         db_namespace["test:load"].invoke
       end
diff --git a/activerecord/lib/active_record/railties/jdbcmysql_error.rb b/activerecord/lib/active_record/railties/jdbcmysql_error.rb
deleted file mode 100644
index 72c75ddd52..0000000000
--- a/activerecord/lib/active_record/railties/jdbcmysql_error.rb
+++ /dev/null
@@ -1,18 +0,0 @@
-# frozen_string_literal: true
-
-#FIXME Remove if ArJdbcMysql will give.
-module ArJdbcMySQL #:nodoc:
-  class Error < StandardError #:nodoc:
-    attr_accessor :error_number, :sql_state
-
-    def initialize(msg)
-      super
-      @error_number = nil
-      @sql_state    = nil
-    end
-
-    # Mysql gem compatibility
-    alias_method :errno, :error_number
-    alias_method :error, :message
-  end
-end
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 889e24dd1a..87bfd75bca 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/string/filters"
-require "active_support/deprecation"
 require "concurrent/map"
 
 module ActiveRecord
@@ -174,11 +173,6 @@ module ActiveRecord
         scope ? [scope] : []
       end
 
-      def scope_chain
-        chain.map(&:scopes)
-      end
-      deprecate :scope_chain
-
       def build_join_constraint(table, foreign_table)
         key         = join_keys.key
         foreign_key = join_keys.foreign_key
@@ -292,13 +286,17 @@ module ActiveRecord
       end
 
       def get_join_keys(association_klass)
-        JoinKeys.new(join_pk(association_klass), join_foreign_key)
+        JoinKeys.new(join_primary_key(association_klass), join_foreign_key)
       end
 
       def build_scope(table, predicate_builder = predicate_builder(table))
         Relation.create(klass, table, predicate_builder)
       end
 
+      def join_primary_key(_)
+        foreign_key
+      end
+
       def join_foreign_key
         active_record_primary_key
       end
@@ -313,10 +311,6 @@ module ActiveRecord
           PredicateBuilder.new(TableMetadata.new(klass, table))
         end
 
-        def join_pk(_)
-          foreign_key
-        end
-
         def primary_key(klass)
           klass.primary_key || raise(UnknownPrimaryKey.new(klass))
         end
@@ -425,21 +419,13 @@ module ActiveRecord
 
       def initialize(name, scope, options, active_record)
         super
-        @automatic_inverse_of = nil
         @type         = options[:as] && (options[:foreign_type] || "#{options[:as]}_type")
-        @foreign_type = options[:foreign_type] || "#{name}_type"
+        @foreign_type = options[:polymorphic] && (options[:foreign_type] || "#{name}_type")
         @constructable = calculate_constructable(macro, options)
         @association_scope_cache = Concurrent::Map.new
 
         if options[:class_name] && options[:class_name].class == Class
-          ActiveSupport::Deprecation.warn(<<-MSG.squish)
-            Passing a class to the `class_name` is deprecated and will raise
-            an ArgumentError in Rails 5.2. It eagerloads more classes than
-            necessary and potentially creates circular dependencies.
-
-            Please pass the class name as a string:
-            `#{macro} :#{name}, class_name: '#{options[:class_name]}'`
-          MSG
+          raise ArgumentError, "A class was passed to `:class_name` but we are expecting a string."
         end
       end
 
@@ -609,12 +595,14 @@ module ActiveRecord
         # If it cannot find a suitable inverse association name, it returns
         # +nil+.
         def inverse_name
-          options.fetch(:inverse_of) do
-            @automatic_inverse_of ||= automatic_inverse_of
+          unless defined?(@inverse_name)
+            @inverse_name = options.fetch(:inverse_of) { automatic_inverse_of }
           end
+
+          @inverse_name
         end
 
-        # returns either false or the inverse association name that it finds.
+        # returns either +nil+ or the inverse association name that it finds.
         def automatic_inverse_of
           if can_find_inverse_of_automatically?(self)
             inverse_name = ActiveSupport::Inflector.underscore(options[:as] || active_record.name.demodulize).to_sym
@@ -631,8 +619,6 @@ module ActiveRecord
               return inverse_name
             end
           end
-
-          false
         end
 
         # Checks if the inverse reflection that is returned from the
@@ -736,6 +722,10 @@ module ActiveRecord
         end
       end
 
+      def join_primary_key(klass)
+        polymorphic? ? association_primary_key(klass) : association_primary_key
+      end
+
       def join_foreign_key
         foreign_key
       end
@@ -745,10 +735,6 @@ module ActiveRecord
         def calculate_constructable(macro, options)
           !polymorphic?
         end
-
-        def join_pk(klass)
-          polymorphic? ? association_primary_key(klass) : association_primary_key
-        end
     end
 
     class HasAndBelongsToManyReflection < AssociationReflection # :nodoc:
@@ -853,10 +839,6 @@ module ActiveRecord
         source_reflection.join_scopes(table, predicate_builder) + super
       end
 
-      def source_type_scope
-        through_reflection.klass.where(foreign_type => options[:source_type])
-      end
-
       def has_scope?
         scope || options[:source_type] ||
           source_reflection.has_scope? ||
@@ -1016,29 +998,24 @@ module ActiveRecord
     end
 
     class PolymorphicReflection < AbstractReflection # :nodoc:
-      delegate :klass, :scope, :plural_name, :type, :get_join_keys, to: :@reflection
+      delegate :klass, :scope, :plural_name, :type, :get_join_keys, :scope_for, to: :@reflection
 
       def initialize(reflection, previous_reflection)
         @reflection = reflection
         @previous_reflection = previous_reflection
       end
 
-      def scopes
-        scopes = @previous_reflection.scopes
-        scopes << @previous_reflection.source_type_scope
-      end
-
       def join_scopes(table, predicate_builder) # :nodoc:
         scopes = @previous_reflection.join_scopes(table, predicate_builder) + super
-        scopes << @previous_reflection.source_type_scope
+        scopes << build_scope(table, predicate_builder).instance_exec(nil, &source_type_scope)
       end
 
       def constraints
-        @reflection.constraints + [source_type_info]
+        @reflection.constraints + [source_type_scope]
       end
 
       private
-        def source_type_info
+        def source_type_scope
           type = @previous_reflection.foreign_type
           source_type = @previous_reflection.options[:source_type]
           lambda { |object| where(type => source_type) }
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 42a9d8492b..081ef5771f 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -36,67 +36,6 @@ module ActiveRecord
       reset
     end
 
-    def insert(values) # :nodoc:
-      primary_key_value = nil
-
-      if primary_key && Hash === values
-        primary_key_value = values[values.keys.find { |k|
-          k.name == primary_key
-        }]
-
-        if !primary_key_value && klass.prefetch_primary_key?
-          primary_key_value = klass.next_sequence_value
-          values[arel_attribute(klass.primary_key)] = primary_key_value
-        end
-      end
-
-      im = arel.create_insert
-      im.into @table
-
-      substitutes = substitute_values values
-
-      if values.empty? # empty insert
-        im.values = Arel.sql(connection.empty_insert_statement_value)
-      else
-        im.insert substitutes
-      end
-
-      @klass.connection.insert(
-        im,
-        "SQL",
-        primary_key || false,
-        primary_key_value,
-        nil,
-      )
-    end
-
-    def _update_record(values, id, id_was) # :nodoc:
-      substitutes = substitute_values values
-
-      scope = @klass.unscoped
-
-      if @klass.finder_needs_type_condition?
-        scope.unscope!(where: @klass.inheritance_column)
-      end
-
-      relation = scope.where(@klass.primary_key => (id_was || id))
-      um = relation
-        .arel
-        .compile_update(substitutes, @klass.primary_key)
-
-      @klass.connection.update(
-        um,
-        "SQL",
-      )
-    end
-
-    def substitute_values(values) # :nodoc:
-      values.map do |arel_attr, value|
-        bind = predicate_builder.build_bind_attribute(arel_attr.name, value)
-        [arel_attr, bind]
-      end
-    end
-
     def arel_attribute(name) # :nodoc:
       klass.arel_attribute(name, table)
     end
@@ -243,9 +182,10 @@ module ActiveRecord
     end
 
     # Converts relation objects to Array.
-    def to_a
+    def to_ary
       records.dup
     end
+    alias to_a to_ary
 
     def records # :nodoc:
       load
@@ -359,6 +299,11 @@ module ActiveRecord
     def update_all(updates)
       raise ArgumentError, "Empty list of attributes to change" if updates.blank?
 
+      if eager_loading?
+        relation = apply_join_dependency
+        return relation.update_all(updates)
+      end
+
       stmt = Arel::UpdateManager.new
 
       stmt.set Arel.sql(@klass.send(:sanitize_sql_for_assignment, updates))
@@ -373,51 +318,7 @@ module ActiveRecord
         stmt.wheres = arel.constraints
       end
 
-      @klass.connection.update stmt, "SQL"
-    end
-
-    # Updates an object (or multiple objects) and saves it to the database, if validations pass.
-    # The resulting object is returned whether the object was saved successfully to the database or not.
-    #
-    # ==== Parameters
-    #
-    # * +id+ - This should be the id or an array of ids to be updated.
-    # * +attributes+ - This should be a hash of attributes or an array of hashes.
-    #
-    # ==== Examples
-    #
-    #   # Updates one record
-    #   Person.update(15, user_name: 'Samuel', group: 'expert')
-    #
-    #   # Updates multiple records
-    #   people = { 1 => { "first_name" => "David" }, 2 => { "first_name" => "Jeremy" } }
-    #   Person.update(people.keys, people.values)
-    #
-    #   # Updates multiple records from the result of a relation
-    #   people = Person.where(group: 'expert')
-    #   people.update(group: 'masters')
-    #
-    # Note: Updating a large number of records will run an
-    # UPDATE query for each record, which may cause a performance
-    # issue. When running callbacks is not needed for each record update,
-    # it is preferred to use #update_all for updating all records
-    # in a single query.
-    def update(id = :all, attributes)
-      if id.is_a?(Array)
-        id.map.with_index { |one_id, idx| update(one_id, attributes[idx]) }
-      elsif id == :all
-        records.each { |record| record.update(attributes) }
-      else
-        if ActiveRecord::Base === id
-          raise ArgumentError, <<-MSG.squish
-            You are passing an instance of ActiveRecord::Base to `update`.
-            Please pass the id of the object by calling `.id`.
-          MSG
-        end
-        object = find(id)
-        object.update(attributes)
-        object
-      end
+      @klass.connection.update stmt, "#{@klass} Update All"
     end
 
     # Destroys the records by instantiating each
@@ -440,33 +341,6 @@ module ActiveRecord
       records.each(&:destroy).tap { reset }
     end
 
-    # Destroy an object (or multiple objects) that has the given id. The object is instantiated first,
-    # therefore all callbacks and filters are fired off before the object is deleted. This method is
-    # less efficient than #delete but allows cleanup methods and other actions to be run.
-    #
-    # This essentially finds the object (or multiple objects) with the given id, creates a new object
-    # from the attributes, and then calls destroy on it.
-    #
-    # ==== Parameters
-    #
-    # * +id+ - Can be either an Integer or an Array of Integers.
-    #
-    # ==== Examples
-    #
-    #   # Destroy a single object
-    #   Todo.destroy(1)
-    #
-    #   # Destroy multiple objects
-    #   todos = [1,2,3]
-    #   Todo.destroy(todos)
-    def destroy(id)
-      if id.is_a?(Array)
-        id.map { |one_id| destroy(one_id) }
-      else
-        find(id).destroy
-      end
-    end
-
     # Deletes the records without instantiating the records
     # first, and hence not calling the {#destroy}[rdoc-ref:Persistence#destroy]
     # method nor invoking callbacks.
@@ -494,6 +368,11 @@ module ActiveRecord
         raise ActiveRecordError.new("delete_all doesn't support #{invalid_methods.join(', ')}")
       end
 
+      if eager_loading?
+        relation = apply_join_dependency
+        return relation.delete_all
+      end
+
       stmt = Arel::DeleteManager.new
       stmt.from(table)
 
@@ -503,35 +382,12 @@ module ActiveRecord
         stmt.wheres = arel.constraints
       end
 
-      affected = @klass.connection.delete(stmt, "SQL")
+      affected = @klass.connection.delete(stmt, "#{@klass} Destroy")
 
       reset
       affected
     end
 
-    # Deletes the row with a primary key matching the +id+ argument, using a
-    # SQL +DELETE+ statement, and returns the number of rows deleted. Active
-    # Record objects are not instantiated, so the object's callbacks are not
-    # executed, including any <tt>:dependent</tt> association options.
-    #
-    # You can delete multiple rows at once by passing an Array of <tt>id</tt>s.
-    #
-    # Note: Although it is often much faster than the alternative,
-    # #destroy, skipping callbacks might bypass business logic in
-    # your application that ensures referential integrity or performs other
-    # essential jobs.
-    #
-    # ==== Examples
-    #
-    #   # Delete a single row
-    #   Todo.delete(1)
-    #
-    #   # Delete multiple rows
-    #   Todo.delete([2,3,4])
-    def delete(id_or_array)
-      where(primary_key => id_or_array).delete_all
-    end
-
     # Causes the records to be loaded from the database if they have not
     # been loaded already. You can use this if for some reason you need
     # to explicitly load some records before actually using them. The
@@ -645,6 +501,11 @@ module ActiveRecord
       limit_value || offset_value
     end
 
+    def alias_tracker(joins = [], aliases = nil) # :nodoc:
+      joins += [aliases] if aliases
+      ActiveRecord::Associations::AliasTracker.create(connection, table.name, joins)
+    end
+
     protected
 
       def load_records(records)
diff --git a/activerecord/lib/active_record/relation/batches.rb b/activerecord/lib/active_record/relation/batches.rb
index 356ad0dcd6..561869017a 100644
--- a/activerecord/lib/active_record/relation/batches.rb
+++ b/activerecord/lib/active_record/relation/batches.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "batches/batch_enumerator"
+require "active_record/relation/batches/batch_enumerator"
 
 module ActiveRecord
   module Batches
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 42d43224fa..d49472fc70 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -130,7 +130,7 @@ module ActiveRecord
     #      end
     def calculate(operation, column_name)
       if has_include?(column_name)
-        relation = construct_relation_for_association_calculations
+        relation = apply_join_dependency
         relation.distinct! if operation.to_s.downcase == "count"
 
         relation.calculate(operation, column_name)
@@ -180,8 +180,10 @@ module ActiveRecord
       end
 
       if has_include?(column_names.first)
-        construct_relation_for_association_calculations.pluck(*column_names)
+        relation = apply_join_dependency
+        relation.pluck(*column_names)
       else
+        enforce_raw_sql_whitelist(column_names)
         relation = spawn
         relation.select_values = column_names.map { |cn|
           @klass.has_attribute?(cn) || @klass.attribute_alias?(cn) ? arel_attribute(cn) : cn
@@ -215,7 +217,7 @@ module ActiveRecord
         if operation == "count"
           column_name ||= select_for_count
           if column_name == :all
-            if distinct && !(has_limit_or_offset? && order_values.any?)
+            if distinct && (group_values.any? || !(has_limit_or_offset? && order_values.any?))
               column_name = primary_key
             end
           elsif column_name =~ /\s*DISTINCT[\s(]+/i
@@ -391,7 +393,7 @@ module ActiveRecord
       def build_count_subquery(relation, column_name, distinct)
         relation.select_values = [
           if column_name == :all
-            distinct ? table[Arel.star] : Arel.sql("1")
+            distinct ? table[Arel.star] : Arel.sql(FinderMethods::ONE_AS_ONE)
           else
             column_alias = Arel.sql("count_column")
             aggregate_column(column_name).as(column_alias)
diff --git a/activerecord/lib/active_record/relation/delegation.rb b/activerecord/lib/active_record/relation/delegation.rb
index 48af777b69..4863befec8 100644
--- a/activerecord/lib/active_record/relation/delegation.rb
+++ b/activerecord/lib/active_record/relation/delegation.rb
@@ -38,7 +38,7 @@ module ActiveRecord
     # may vary depending on the klass of a relation, so we create a subclass of Relation
     # for each different klass, and the delegations are compiled into that subclass only.
 
-    delegate :to_xml, :encode_with, :length, :each, :uniq, :to_ary, :join,
+    delegate :to_xml, :encode_with, :length, :each, :uniq, :join,
              :[], :&, :|, :+, :-, :sample, :reverse, :rotate, :compact, :in_groups, :in_groups_of,
              :to_sentence, :to_formatted_s, :as_json,
              :shuffle, :split, :slice, :index, :rindex, to: :records
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index c92d5a52f4..ff06ecbee1 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -18,9 +18,10 @@ module ActiveRecord
     #   Person.find([1])        # returns an array for the object with ID = 1
     #   Person.where("administrator = 1").order("created_on DESC").find(1)
     #
-    # NOTE: The returned records may not be in the same order as the ids you
-    # provide since database rows are unordered. You will need to provide an explicit QueryMethods#order
-    # option if you want the results to be sorted.
+    # NOTE: The returned records are in the same order as the ids you provide.
+    # If you want the results to be sorted by database, you can use ActiveRecord::QueryMethods#where
+    # method and provide an explicit ActiveRecord::QueryMethods#order option.
+    # But ActiveRecord::QueryMethods#where method doesn't raise ActiveRecord::RecordNotFound.
     #
     # ==== Find with lock
     #
@@ -88,7 +89,7 @@ module ActiveRecord
       where(arg, *args).take!
     rescue ::RangeError
       raise RecordNotFound.new("Couldn't find #{@klass.name} with an out of range value",
-                               @klass.name)
+                               @klass.name, @klass.primary_key)
     end
 
     # Gives a record (or N records if a parameter is supplied) without any implied
@@ -284,7 +285,7 @@ module ActiveRecord
     # * Hash - Finds the record that matches these +find+-style conditions
     #   (such as <tt>{name: 'David'}</tt>).
     # * +false+ - Returns always +false+.
-    # * No args - Returns +false+ if the table is empty, +true+ otherwise.
+    # * No args - Returns +false+ if the relation is empty, +true+ otherwise.
     #
     # For more information about specifying conditions as a hash or array,
     # see the Conditions section in the introduction to ActiveRecord::Base.
@@ -300,6 +301,7 @@ module ActiveRecord
     #   Person.exists?(name: 'David')
     #   Person.exists?(false)
     #   Person.exists?
+    #   Person.where(name: 'Spartacus', rating: 4).exists?
     def exists?(conditions = :none)
       if Base === conditions
         raise ArgumentError, <<-MSG.squish
@@ -310,12 +312,12 @@ module ActiveRecord
 
       return false if !conditions || limit_value == 0
 
-      relation = self unless eager_loading?
-      relation ||= apply_join_dependency(self, construct_join_dependency(eager_loading: false))
-
-      return false if ActiveRecord::NullRelation === relation
+      if eager_loading?
+        relation = apply_join_dependency(construct_join_dependency(eager_loading: false))
+        return relation.exists?(conditions)
+      end
 
-      relation = construct_relation_for_exists(relation, conditions)
+      relation = construct_relation_for_exists(conditions)
 
       skip_query_cache_if_necessary { connection.select_value(relation.arel, "#{name} Exists") } ? true : false
     rescue ::RangeError
@@ -338,7 +340,7 @@ module ActiveRecord
       if ids.nil?
         error = "Couldn't find #{name}".dup
         error << " with#{conditions}" if conditions
-        raise RecordNotFound.new(error, name)
+        raise RecordNotFound.new(error, name, key)
       elsif Array(ids).size == 1
         error = "Couldn't find #{name} with '#{key}'=#{ids}#{conditions}"
         raise RecordNotFound.new(error, name, key, ids)
@@ -346,7 +348,7 @@ module ActiveRecord
         error = "Couldn't find all #{name.pluralize} with '#{key}': ".dup
         error << "(#{ids.join(", ")})#{conditions} (found #{result_size} results, but was looking for #{expected_size})."
         error << " Couldn't find #{name.pluralize(not_found_ids.size)} with #{key.to_s.pluralize(not_found_ids.size)} #{not_found_ids.join(', ')}." if not_found_ids
-        raise RecordNotFound.new(error, name, primary_key, ids)
+        raise RecordNotFound.new(error, name, key, ids)
       end
     end
 
@@ -366,17 +368,16 @@ module ActiveRecord
         # preexisting join in joins_values to categorizations (by way of
         # the `has_many :through` for categories).
         #
-        join_dependency = construct_join_dependency(joins_values)
+        join_dependency = construct_join_dependency
 
-        aliases  = join_dependency.aliases
-        relation = select aliases.columns
-        relation = apply_join_dependency(relation, join_dependency)
+        relation = apply_join_dependency(join_dependency)
+        relation._select!(join_dependency.aliases.columns)
 
         yield relation, join_dependency
       end
 
-      def construct_relation_for_exists(relation, conditions)
-        relation = relation.except(:select, :distinct, :order)._select!(ONE_AS_ONE).limit!(1)
+      def construct_relation_for_exists(conditions)
+        relation = except(:select, :distinct, :order)._select!(ONE_AS_ONE).limit!(1)
 
         case conditions
         when Array, Hash
@@ -388,17 +389,15 @@ module ActiveRecord
         relation
       end
 
-      def construct_join_dependency(joins = [], eager_loading: true)
+      def construct_join_dependency(eager_loading: true)
         including = eager_load_values + includes_values
-        ActiveRecord::Associations::JoinDependency.new(klass, table, including, joins, eager_loading: eager_loading)
-      end
-
-      def construct_relation_for_association_calculations
-        apply_join_dependency(self, construct_join_dependency(joins_values))
+        ActiveRecord::Associations::JoinDependency.new(
+          klass, table, including, alias_tracker(joins_values), eager_loading: eager_loading
+        )
       end
 
-      def apply_join_dependency(relation, join_dependency)
-        relation = relation.except(:includes, :eager_load, :preload).joins!(join_dependency)
+      def apply_join_dependency(join_dependency = construct_join_dependency)
+        relation = except(:includes, :eager_load, :preload).joins!(join_dependency)
 
         if using_limitable_reflections?(join_dependency.reflections)
           relation
@@ -435,9 +434,12 @@ module ActiveRecord
 
         ids = ids.flatten.compact.uniq
 
+        model_name = @klass.name
+
         case ids.size
         when 0
-          raise RecordNotFound, "Couldn't find #{@klass.name} without an ID"
+          error_message = "Couldn't find #{model_name} without an ID"
+          raise RecordNotFound.new(error_message, model_name, primary_key)
         when 1
           result = find_one(ids.first)
           expects_array ? [ result ] : result
@@ -445,7 +447,8 @@ module ActiveRecord
           find_some(ids)
         end
       rescue ::RangeError
-        raise RecordNotFound, "Couldn't find #{@klass.name} with an out of range ID"
+        error_message = "Couldn't find #{model_name} with an out of range ID"
+        raise RecordNotFound.new(error_message, model_name, primary_key, ids)
       end
 
       def find_one(id)
diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 03824ffff9..b736b21525 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -112,22 +112,20 @@ module ActiveRecord
           if other.klass == relation.klass
             relation.joins!(*other.joins_values)
           else
-            joins_dependency, rest = other.joins_values.partition do |join|
+            alias_tracker = nil
+            joins_dependency = other.joins_values.map do |join|
               case join
               when Hash, Symbol, Array
-                true
+                alias_tracker ||= other.alias_tracker
+                ActiveRecord::Associations::JoinDependency.new(
+                  other.klass, other.table, join, alias_tracker
+                )
               else
-                false
+                join
               end
             end
 
-            join_dependency = ActiveRecord::Associations::JoinDependency.new(
-              other.klass, other.table, joins_dependency, []
-            )
-
-            relation.joins! rest
-
-            @relation = relation.joins join_dependency
+            relation.joins!(*joins_dependency)
           end
         end
 
diff --git a/activerecord/lib/active_record/relation/predicate_builder.rb b/activerecord/lib/active_record/relation/predicate_builder.rb
index 5c42414072..885c26d7aa 100644
--- a/activerecord/lib/active_record/relation/predicate_builder.rb
+++ b/activerecord/lib/active_record/relation/predicate_builder.rb
@@ -13,6 +13,7 @@ module ActiveRecord
       register_handler(Range, RangeHandler.new(self))
       register_handler(Relation, RelationHandler.new)
       register_handler(Array, ArrayHandler.new(self))
+      register_handler(Set, ArrayHandler.new(self))
     end
 
     def build_from_hash(attributes)
@@ -123,11 +124,11 @@ module ActiveRecord
   end
 end
 
-require_relative "predicate_builder/array_handler"
-require_relative "predicate_builder/base_handler"
-require_relative "predicate_builder/basic_object_handler"
-require_relative "predicate_builder/range_handler"
-require_relative "predicate_builder/relation_handler"
+require "active_record/relation/predicate_builder/array_handler"
+require "active_record/relation/predicate_builder/base_handler"
+require "active_record/relation/predicate_builder/basic_object_handler"
+require "active_record/relation/predicate_builder/range_handler"
+require "active_record/relation/predicate_builder/relation_handler"
 
-require_relative "predicate_builder/association_query_value"
-require_relative "predicate_builder/polymorphic_array_value"
+require "active_record/relation/predicate_builder/association_query_value"
+require "active_record/relation/predicate_builder/polymorphic_array_value"
diff --git a/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb b/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb
index f51ea4fde0..c8bbfa5051 100644
--- a/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb
+++ b/activerecord/lib/active_record/relation/predicate_builder/relation_handler.rb
@@ -4,6 +4,10 @@ module ActiveRecord
   class PredicateBuilder
     class RelationHandler # :nodoc:
       def call(attribute, value)
+        if value.eager_loading?
+          value = value.send(:apply_join_dependency)
+        end
+
         if value.select_values.empty?
           value = value.select(value.arel_attribute(value.klass.primary_key))
         end
diff --git a/activerecord/lib/active_record/relation/query_attribute.rb b/activerecord/lib/active_record/relation/query_attribute.rb
index 5a9a7fd432..3532f28858 100644
--- a/activerecord/lib/active_record/relation/query_attribute.rb
+++ b/activerecord/lib/active_record/relation/query_attribute.rb
@@ -1,10 +1,10 @@
 # frozen_string_literal: true
 
-require_relative "../attribute"
+require "active_model/attribute"
 
 module ActiveRecord
   class Relation
-    class QueryAttribute < Attribute # :nodoc:
+    class QueryAttribute < ActiveModel::Attribute # :nodoc:
       def type_cast(value)
         value
       end
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index c88603fde2..749223422f 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
-require_relative "from_clause"
-require_relative "query_attribute"
-require_relative "where_clause"
-require_relative "where_clause_factory"
+require "active_record/relation/from_clause"
+require "active_record/relation/query_attribute"
+require "active_record/relation/where_clause"
+require "active_record/relation/where_clause_factory"
 require "active_model/forbidden_attributes_protection"
 
 module ActiveRecord
@@ -295,6 +295,7 @@ module ActiveRecord
       spawn.order!(*args)
     end
 
+    # Same as #order but operates on relation in-place instead of copying.
     def order!(*args) # :nodoc:
       preprocess_order_args(args)
 
@@ -316,6 +317,7 @@ module ActiveRecord
       spawn.reorder!(*args)
     end
 
+    # Same as #reorder but operates on relation in-place instead of copying.
     def reorder!(*args) # :nodoc:
       preprocess_order_args(args)
 
@@ -441,7 +443,7 @@ module ActiveRecord
     #   => SELECT "users".* FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
     #
     def left_outer_joins(*args)
-      check_if_method_has_arguments!(:left_outer_joins, args)
+      check_if_method_has_arguments!(__callee__, args)
 
       args.compact!
       args.flatten!
@@ -898,8 +900,8 @@ module ActiveRecord
     end
 
     # Returns the Arel object associated with the relation.
-    def arel # :nodoc:
-      @arel ||= build_arel
+    def arel(aliases = nil) # :nodoc:
+      @arel ||= build_arel(aliases)
     end
 
     protected
@@ -921,16 +923,16 @@ module ActiveRecord
         raise ImmutableRelation if defined?(@arel) && @arel
       end
 
-      def build_arel
+      def build_arel(aliases)
         arel = Arel::SelectManager.new(table)
 
-        build_joins(arel, joins_values.flatten) unless joins_values.empty?
-        build_left_outer_joins(arel, left_outer_joins_values.flatten) unless left_outer_joins_values.empty?
+        aliases = build_joins(arel, joins_values.flatten, aliases) unless joins_values.empty?
+        build_left_outer_joins(arel, left_outer_joins_values.flatten, aliases) unless left_outer_joins_values.empty?
 
         arel.where(where_clause.ast) unless where_clause.empty?
         arel.having(having_clause.ast) unless having_clause.empty?
         if limit_value
-          limit_attribute = Attribute.with_cast_value(
+          limit_attribute = ActiveModel::Attribute.with_cast_value(
             "LIMIT".freeze,
             connection.sanitize_limit(limit_value),
             Type.default_value,
@@ -938,7 +940,7 @@ module ActiveRecord
           arel.take(Arel::Nodes::BindParam.new(limit_attribute))
         end
         if offset_value
-          offset_attribute = Attribute.with_cast_value(
+          offset_attribute = ActiveModel::Attribute.with_cast_value(
             "OFFSET".freeze,
             offset_value.to_i,
             Type.default_value,
@@ -963,6 +965,9 @@ module ActiveRecord
         name = from_clause.name
         case opts
         when Relation
+          if opts.eager_loading?
+            opts = opts.send(:apply_join_dependency)
+          end
           name ||= "subquery"
           opts.arel.as(name.to_s)
         else
@@ -970,7 +975,7 @@ module ActiveRecord
         end
       end
 
-      def build_left_outer_joins(manager, outer_joins)
+      def build_left_outer_joins(manager, outer_joins, aliases)
         buckets = outer_joins.group_by do |join|
           case join
           when Hash, Symbol, Array
@@ -980,10 +985,10 @@ module ActiveRecord
           end
         end
 
-        build_join_query(manager, buckets, Arel::Nodes::OuterJoin)
+        build_join_query(manager, buckets, Arel::Nodes::OuterJoin, aliases)
       end
 
-      def build_joins(manager, joins)
+      def build_joins(manager, joins, aliases)
         buckets = joins.group_by do |join|
           case join
           when String
@@ -999,10 +1004,10 @@ module ActiveRecord
           end
         end
 
-        build_join_query(manager, buckets, Arel::Nodes::InnerJoin)
+        build_join_query(manager, buckets, Arel::Nodes::InnerJoin, aliases)
       end
 
-      def build_join_query(manager, buckets, join_type)
+      def build_join_query(manager, buckets, join_type, aliases)
         buckets.default = []
 
         association_joins         = buckets[:association_join]
@@ -1011,9 +1016,10 @@ module ActiveRecord
         string_joins              = buckets[:string_join].map(&:strip).uniq
 
         join_list = join_nodes + convert_join_strings_to_ast(manager, string_joins)
+        alias_tracker = alias_tracker(join_list, aliases)
 
         join_dependency = ActiveRecord::Associations::JoinDependency.new(
-          klass, table, association_joins, join_list
+          klass, table, association_joins, alias_tracker
         )
 
         joins = join_dependency.join_constraints(stashed_association_joins, join_type)
@@ -1021,7 +1027,7 @@ module ActiveRecord
 
         manager.join_sources.concat(join_list)
 
-        manager
+        alias_tracker.aliases
       end
 
       def convert_join_strings_to_ast(table, joins)
@@ -1034,6 +1040,8 @@ module ActiveRecord
       def build_select(arel)
         if select_values.any?
           arel.project(*arel_columns(select_values.uniq))
+        elsif @klass.ignored_columns.any?
+          arel.project(*arel_columns(@klass.column_names))
         else
           arel.project(table[Arel.star])
         end
@@ -1070,7 +1078,7 @@ module ActiveRecord
             end
             o.split(",").map! do |s|
               s.strip!
-              s.gsub!(/\sasc\Z/i, " DESC") || s.gsub!(/\sdesc\Z/i, " ASC") || s.concat(" DESC")
+              s.gsub!(/\sasc\Z/i, " DESC") || s.gsub!(/\sdesc\Z/i, " ASC") || (s << " DESC")
             end
           else
             o
@@ -1079,6 +1087,10 @@ module ActiveRecord
       end
 
       def does_not_support_reverse?(order)
+        # Account for String subclasses like Arel::Nodes::SqlLiteral that
+        # override methods like #count.
+        order = String.new(order) unless order.instance_of?(String)
+
         # Uses SQL function with multiple arguments.
         (order.include?(",") && order.split(",").find { |section| section.count("(") != section.count(")") }) ||
           # Uses "nulls first" like construction.
@@ -1112,6 +1124,12 @@ module ActiveRecord
           klass.send(:sanitize_sql_for_order, arg)
         end
         order_args.flatten!
+
+        @klass.enforce_raw_sql_whitelist(
+          order_args.flat_map { |a| a.is_a?(Hash) ? a.keys : a },
+          whitelist: AttributeMethods::ClassMethods::COLUMN_NAME_ORDER_WHITELIST
+        )
+
         validate_order_args(order_args)
 
         references = order_args.grep(String)
diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index 424894f835..617d8de8b2 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -2,7 +2,7 @@
 
 require "active_support/core_ext/hash/except"
 require "active_support/core_ext/hash/slice"
-require_relative "merger"
+require "active_record/relation/merger"
 
 module ActiveRecord
   module SpawnMethods
diff --git a/activerecord/lib/active_record/relation/where_clause.rb b/activerecord/lib/active_record/relation/where_clause.rb
index 752bb38481..a502713e56 100644
--- a/activerecord/lib/active_record/relation/where_clause.rb
+++ b/activerecord/lib/active_record/relation/where_clause.rb
@@ -47,7 +47,7 @@ module ActiveRecord
       end
 
       def to_h(table_name = nil)
-        equalities = predicates.grep(Arel::Nodes::Equality)
+        equalities = equalities(predicates)
         if table_name
           equalities = equalities.select do |node|
             node.left.relation.name == table_name
@@ -90,6 +90,20 @@ module ActiveRecord
         end
 
       private
+        def equalities(predicates)
+          equalities = []
+
+          predicates.each do |node|
+            case node
+            when Arel::Nodes::Equality
+              equalities << node
+            when Arel::Nodes::And
+              equalities.concat equalities(node.children)
+            end
+          end
+
+          equalities
+        end
 
         def predicates_unreferenced_by(other)
           predicates.reject do |n|
@@ -121,7 +135,7 @@ module ActiveRecord
         end
 
         def except_predicates(columns)
-          self.predicates.reject do |node|
+          predicates.reject do |node|
             case node
             when Arel::Nodes::Between, Arel::Nodes::In, Arel::Nodes::NotIn, Arel::Nodes::Equality, Arel::Nodes::NotEqual, Arel::Nodes::LessThan, Arel::Nodes::LessThanOrEqual, Arel::Nodes::GreaterThan, Arel::Nodes::GreaterThanOrEqual
               subrelation = (node.left.kind_of?(Arel::Attributes::Attribute) ? node.left : node.right)
diff --git a/activerecord/lib/active_record/sanitization.rb b/activerecord/lib/active_record/sanitization.rb
index 91a4f1fad6..21f8bc7cb2 100644
--- a/activerecord/lib/active_record/sanitization.rb
+++ b/activerecord/lib/active_record/sanitization.rb
@@ -30,8 +30,6 @@ module ActiveRecord
           end
         end
         alias :sanitize_sql :sanitize_sql_for_conditions
-        alias :sanitize_conditions :sanitize_sql
-        deprecate sanitize_conditions: :sanitize_sql
 
         # Accepts an array, hash, or string of SQL conditions and sanitizes
         # them into a valid SQL fragment for a SET clause.
@@ -65,7 +63,17 @@ module ActiveRecord
         #   # => "id ASC"
         def sanitize_sql_for_order(condition) # :doc:
           if condition.is_a?(Array) && condition.first.to_s.include?("?")
-            sanitize_sql_array(condition)
+            enforce_raw_sql_whitelist([condition.first],
+              whitelist: AttributeMethods::ClassMethods::COLUMN_NAME_ORDER_WHITELIST
+            )
+
+            # Ensure we aren't dealing with a subclass of String that might
+            # override methods we use (eg. Arel::Nodes::SqlLiteral).
+            if condition.first.kind_of?(String) && !condition.first.instance_of?(String)
+              condition = [String.new(condition.first), *condition[1..-1]]
+            end
+
+            Arel.sql(sanitize_sql_array(condition))
           else
             condition
           end
@@ -112,7 +120,8 @@ module ActiveRecord
         def sanitize_sql_hash_for_assignment(attrs, table) # :doc:
           c = connection
           attrs.map do |attr, value|
-            value = type_for_attribute(attr.to_s).serialize(value)
+            type = type_for_attribute(attr.to_s)
+            value = type.serialize(type.cast(value))
             "#{c.quote_table_name_for_assignment(table, attr)} = #{c.quote(value)}"
           end.join(", ")
         end
@@ -207,10 +216,5 @@ module ActiveRecord
           end
         end
     end
-
-    def quoted_id # :nodoc:
-      self.class.connection.quote(@attributes[self.class.primary_key].value_for_database)
-    end
-    deprecate :quoted_id
   end
 end
diff --git a/activerecord/lib/active_record/schema_dumper.rb b/activerecord/lib/active_record/schema_dumper.rb
index 8d0311fabd..16ccba6b6c 100644
--- a/activerecord/lib/active_record/schema_dumper.rb
+++ b/activerecord/lib/active_record/schema_dumper.rb
@@ -82,16 +82,8 @@ HEADER
         stream.puts "end"
       end
 
+      # extensions are only supported by PostgreSQL
       def extensions(stream)
-        return unless @connection.supports_extensions?
-        extensions = @connection.extensions
-        if extensions.any?
-          stream.puts "  # These are extensions that must be enabled in order to support this database"
-          extensions.sort.each do |extension|
-            stream.puts "  enable_extension #{extension.inspect}"
-          end
-          stream.puts
-        end
       end
 
       def tables(stream)
@@ -192,8 +184,9 @@ HEADER
           "name: #{index.name.inspect}",
         ]
         index_parts << "unique: true" if index.unique
-        index_parts << "length: { #{format_options(index.lengths)} }" if index.lengths.present?
-        index_parts << "order: { #{format_options(index.orders)} }" if index.orders.present?
+        index_parts << "length: #{format_index_parts(index.lengths)}" if index.lengths.present?
+        index_parts << "order: #{format_index_parts(index.orders)}" if index.orders.present?
+        index_parts << "opclass: #{format_index_parts(index.opclasses)}" if index.opclasses.present?
         index_parts << "where: #{index.where.inspect}" if index.where
         index_parts << "using: #{index.using.inspect}" if !@connection.default_index_type?(index)
         index_parts << "type: #{index.type.inspect}" if index.type
@@ -239,6 +232,14 @@ HEADER
         options.map { |key, value| "#{key}: #{value.inspect}" }.join(", ")
       end
 
+      def format_index_parts(options)
+        if options.is_a?(Hash)
+          "{ #{format_options(options)} }"
+        else
+          options.inspect
+        end
+      end
+
       def remove_prefix_and_suffix(table)
         prefix = Regexp.escape(@options[:table_name_prefix].to_s)
         suffix = Regexp.escape(@options[:table_name_suffix].to_s)
diff --git a/activerecord/lib/active_record/schema_migration.rb b/activerecord/lib/active_record/schema_migration.rb
index 339a5334a8..f2d8b038fa 100644
--- a/activerecord/lib/active_record/schema_migration.rb
+++ b/activerecord/lib/active_record/schema_migration.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require_relative "scoping/default"
-require_relative "scoping/named"
+require "active_record/scoping/default"
+require "active_record/scoping/named"
 
 module ActiveRecord
   # This class is used to create a table that keeps track of which migrations
diff --git a/activerecord/lib/active_record/scoping.rb b/activerecord/lib/active_record/scoping.rb
index da585a9562..01ac56570a 100644
--- a/activerecord/lib/active_record/scoping.rb
+++ b/activerecord/lib/active_record/scoping.rb
@@ -11,23 +11,23 @@ module ActiveRecord
       include Named
     end
 
-    module ClassMethods
-      def current_scope(skip_inherited_scope = false) # :nodoc:
+    module ClassMethods # :nodoc:
+      def current_scope(skip_inherited_scope = false)
         ScopeRegistry.value_for(:current_scope, self, skip_inherited_scope)
       end
 
-      def current_scope=(scope) #:nodoc:
+      def current_scope=(scope)
         ScopeRegistry.set_value_for(:current_scope, self, scope)
       end
 
       # Collects attributes from scopes that should be applied when creating
       # an AR instance for the particular class this is called on.
-      def scope_attributes # :nodoc:
+      def scope_attributes
         all.scope_for_create
       end
 
       # Are there attributes associated with this scope?
-      def scope_attributes? # :nodoc:
+      def scope_attributes?
         current_scope
       end
     end
diff --git a/activerecord/lib/active_record/scoping/default.rb b/activerecord/lib/active_record/scoping/default.rb
index 86ae374318..8c612df27a 100644
--- a/activerecord/lib/active_record/scoping/default.rb
+++ b/activerecord/lib/active_record/scoping/default.rb
@@ -111,7 +111,7 @@ module ActiveRecord
               # The user has defined their own default scope method, so call that
               evaluate_default_scope do
                 if scope = default_scope
-                  (base_rel ||= relation).merge(scope)
+                  (base_rel ||= relation).merge!(scope)
                 end
               end
             elsif default_scopes.any?
@@ -119,7 +119,7 @@ module ActiveRecord
               evaluate_default_scope do
                 default_scopes.inject(base_rel) do |default_scope, scope|
                   scope = scope.respond_to?(:to_proc) ? scope : scope.method(:call)
-                  default_scope.merge(base_rel.instance_exec(&scope))
+                  default_scope.merge!(base_rel.instance_exec(&scope))
                 end
               end
             end
diff --git a/activerecord/lib/active_record/scoping/named.rb b/activerecord/lib/active_record/scoping/named.rb
index 6fa096c1fe..752655aa05 100644
--- a/activerecord/lib/active_record/scoping/named.rb
+++ b/activerecord/lib/active_record/scoping/named.rb
@@ -24,8 +24,14 @@ module ActiveRecord
         # You can define a scope that applies to all finders using
         # {default_scope}[rdoc-ref:Scoping::Default::ClassMethods#default_scope].
         def all
+          current_scope = self.current_scope
+
           if current_scope
-            current_scope.clone
+            if self == current_scope.klass
+              current_scope.clone
+            else
+              relation.merge!(current_scope)
+            end
           else
             default_scoped
           end
@@ -165,6 +171,12 @@ module ActiveRecord
               "a class method with the same name."
           end
 
+          if method_defined_within?(name, Relation)
+            raise ArgumentError, "You tried to define a scope named \"#{name}\" " \
+              "on the model \"#{self.name}\", but ActiveRecord::Relation already defined " \
+              "an instance method with the same name."
+          end
+
           valid_scope_name?(name)
           extension = Module.new(&block) if block
 
diff --git a/activerecord/lib/active_record/tasks/database_tasks.rb b/activerecord/lib/active_record/tasks/database_tasks.rb
index 0f3f84ca08..4657e51e6d 100644
--- a/activerecord/lib/active_record/tasks/database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/database_tasks.rb
@@ -164,13 +164,12 @@ module ActiveRecord
       end
 
       def migrate
-        raise "Empty VERSION provided" if ENV["VERSION"] && ENV["VERSION"].empty?
+        check_target_version
 
         verbose = ENV["VERBOSE"] ? ENV["VERBOSE"] != "false" : true
-        version = ENV["VERSION"] ? ENV["VERSION"].to_i : nil
         scope = ENV["SCOPE"]
         verbose_was, Migration.verbose = Migration.verbose, verbose
-        Migrator.migrate(migrations_paths, version) do |migration|
+        Migrator.migrate(migrations_paths, target_version) do |migration|
           scope.blank? || scope == migration.scope
         end
         ActiveRecord::Base.clear_cache!
@@ -178,6 +177,16 @@ module ActiveRecord
         Migration.verbose = verbose_was
       end
 
+      def check_target_version
+        if target_version && !(Migration::MigrationFilenameRegexp.match?(ENV["VERSION"]) || /\A\d+\z/.match?(ENV["VERSION"]))
+          raise "Invalid format of target version: `VERSION=#{ENV['VERSION']}`"
+        end
+      end
+
+      def target_version
+        ENV["VERSION"].to_i if ENV["VERSION"] && !ENV["VERSION"].empty?
+      end
+
       def charset_current(environment = env)
         charset ActiveRecord::Base.configurations[environment]
       end
@@ -225,22 +234,22 @@ module ActiveRecord
         class_for_adapter(configuration["adapter"]).new(*arguments).structure_load(filename, structure_load_flags)
       end
 
-      def load_schema(configuration, format = ActiveRecord::Base.schema_format, file = nil) # :nodoc:
+      def load_schema(configuration, format = ActiveRecord::Base.schema_format, file = nil, environment = env) # :nodoc:
         file ||= schema_file(format)
 
+        check_schema_file(file)
+        ActiveRecord::Base.establish_connection(configuration)
+
         case format
         when :ruby
-          check_schema_file(file)
-          ActiveRecord::Base.establish_connection(configuration)
           load(file)
         when :sql
-          check_schema_file(file)
           structure_load(configuration, file)
         else
           raise ArgumentError, "unknown format #{format.inspect}"
         end
         ActiveRecord::InternalMetadata.create_table
-        ActiveRecord::InternalMetadata[:environment] = ActiveRecord::Migrator.current_environment
+        ActiveRecord::InternalMetadata[:environment] = environment
       end
 
       def schema_file(format = ActiveRecord::Base.schema_format)
@@ -253,8 +262,8 @@ module ActiveRecord
       end
 
       def load_schema_current(format = ActiveRecord::Base.schema_format, file = nil, environment = env)
-        each_current_configuration(environment) { |configuration|
-          load_schema configuration, format, file
+        each_current_configuration(environment) { |configuration, configuration_environment|
+          load_schema configuration, format, file, configuration_environment
         }
         ActiveRecord::Base.establish_connection(environment.to_sym)
       end
@@ -301,9 +310,10 @@ module ActiveRecord
           environments = [environment]
           environments << "test" if environment == "development"
 
-          configurations = ActiveRecord::Base.configurations.values_at(*environments)
-          configurations.compact.each do |configuration|
-            yield configuration unless configuration["database"].blank?
+          ActiveRecord::Base.configurations.slice(*environments).each do |configuration_environment, configuration|
+            next unless configuration["database"]
+
+            yield configuration, configuration_environment
           end
         end
 
diff --git a/activerecord/lib/active_record/tasks/mysql_database_tasks.rb b/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
index 84265aa9e3..e697fa6def 100644
--- a/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
@@ -3,8 +3,6 @@
 module ActiveRecord
   module Tasks # :nodoc:
     class MySQLDatabaseTasks # :nodoc:
-      ACCESS_DENIED_ERROR = 1045
-
       delegate :connection, :establish_connection, to: ActiveRecord::Base
 
       def initialize(configuration)
@@ -21,20 +19,6 @@ module ActiveRecord
         else
           raise
         end
-      rescue error_class => error
-        if error.respond_to?(:errno) && error.errno == ACCESS_DENIED_ERROR
-          $stdout.print error.message
-          establish_connection root_configuration_without_database
-          connection.create_database configuration["database"], creation_options
-          if configuration["username"] != "root"
-            connection.execute grant_statement.gsub(/\s+/, " ").strip
-          end
-          establish_connection configuration
-        else
-          $stderr.puts error.inspect
-          $stderr.puts "Couldn't create database for #{configuration.inspect}, #{creation_options.inspect}"
-          $stderr.puts "(If you set the charset manually, make sure you have a matching collation)" if configuration["encoding"]
-        end
       end
 
       def drop
@@ -99,37 +83,6 @@ module ActiveRecord
           end
         end
 
-        def error_class
-          if configuration["adapter"].include?("jdbc")
-            require_relative "../railties/jdbcmysql_error"
-            ArJdbcMySQL::Error
-          elsif defined?(Mysql2)
-            Mysql2::Error
-          else
-            StandardError
-          end
-        end
-
-        def grant_statement
-          <<-SQL
-GRANT ALL PRIVILEGES ON `#{configuration['database']}`.*
-  TO '#{configuration['username']}'@'localhost'
-IDENTIFIED BY '#{configuration['password']}' WITH GRANT OPTION;
-          SQL
-        end
-
-        def root_configuration_without_database
-          configuration_without_database.merge(
-            "username" => "root",
-            "password" => root_password
-          )
-        end
-
-        def root_password
-          $stdout.print "Please provide the root password for your MySQL installation\n>"
-          $stdin.gets.strip
-        end
-
         def prepare_command_options
           args = {
             "host"      => "--host",
diff --git a/activerecord/lib/active_record/transactions.rb b/activerecord/lib/active_record/transactions.rb
index 6761f2da25..97cba5d1c7 100644
--- a/activerecord/lib/active_record/transactions.rb
+++ b/activerecord/lib/active_record/transactions.rb
@@ -285,7 +285,7 @@ module ActiveRecord
             fire_on = Array(options[:on])
             assert_valid_transaction_action(fire_on)
             options[:if] = Array(options[:if])
-            options[:if].unshift("transaction_include_any_action?(#{fire_on})")
+            options[:if].unshift(-> { transaction_include_any_action?(fire_on) })
           end
         end
 
diff --git a/activerecord/lib/active_record/type.rb b/activerecord/lib/active_record/type.rb
index fa22df92b8..c303186ef2 100644
--- a/activerecord/lib/active_record/type.rb
+++ b/activerecord/lib/active_record/type.rb
@@ -2,21 +2,21 @@
 
 require "active_model/type"
 
-require_relative "type/internal/timezone"
+require "active_record/type/internal/timezone"
 
-require_relative "type/date"
-require_relative "type/date_time"
-require_relative "type/decimal_without_scale"
-require_relative "type/json"
-require_relative "type/time"
-require_relative "type/text"
-require_relative "type/unsigned_integer"
+require "active_record/type/date"
+require "active_record/type/date_time"
+require "active_record/type/decimal_without_scale"
+require "active_record/type/json"
+require "active_record/type/time"
+require "active_record/type/text"
+require "active_record/type/unsigned_integer"
 
-require_relative "type/serialized"
-require_relative "type/adapter_specific_registry"
+require "active_record/type/serialized"
+require "active_record/type/adapter_specific_registry"
 
-require_relative "type/type_map"
-require_relative "type/hash_lookup_type_map"
+require "active_record/type/type_map"
+require "active_record/type/hash_lookup_type_map"
 
 module ActiveRecord
   module Type
diff --git a/activerecord/lib/active_record/type_caster.rb b/activerecord/lib/active_record/type_caster.rb
index ed2e4fb79c..2e5f45fa3d 100644
--- a/activerecord/lib/active_record/type_caster.rb
+++ b/activerecord/lib/active_record/type_caster.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require_relative "type_caster/map"
-require_relative "type_caster/connection"
+require "active_record/type_caster/map"
+require "active_record/type_caster/connection"
 
 module ActiveRecord
   module TypeCaster # :nodoc:
diff --git a/activerecord/lib/active_record/validations.rb b/activerecord/lib/active_record/validations.rb
index 3f5c879f2f..ca27a3f0ab 100644
--- a/activerecord/lib/active_record/validations.rb
+++ b/activerecord/lib/active_record/validations.rb
@@ -86,8 +86,8 @@ module ActiveRecord
   end
 end
 
-require_relative "validations/associated"
-require_relative "validations/uniqueness"
-require_relative "validations/presence"
-require_relative "validations/absence"
-require_relative "validations/length"
+require "active_record/validations/associated"
+require "active_record/validations/uniqueness"
+require "active_record/validations/presence"
+require "active_record/validations/absence"
+require "active_record/validations/length"
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index f4ad58c087..4c2c5dd852 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -205,9 +205,7 @@ module ActiveRecord
       #                                      | # Boom! We now have a duplicate
       #                                      | # title!
       #
-      # This could even happen if you use transactions with the 'serializable'
-      # isolation level. The best way to work around this problem is to add a unique
-      # index to the database table using
+      # The best way to work around this problem is to add a unique index to the database table using
       # {connection.add_index}[rdoc-ref:ConnectionAdapters::SchemaStatements#add_index].
       # In the rare case that a race condition occurs, the database will guarantee
       # the field's uniqueness.
diff --git a/activerecord/lib/rails/generators/active_record/application_record/templates/application_record.rb b/activerecord/lib/rails/generators/active_record/application_record/templates/application_record.rb.tt
index 60050e0bf8..60050e0bf8 100644
--- a/activerecord/lib/rails/generators/active_record/application_record/templates/application_record.rb
+++ b/activerecord/lib/rails/generators/active_record/application_record/templates/application_record.rb.tt
diff --git a/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb b/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb.tt
index 5f7201cfe1..5f7201cfe1 100644
--- a/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb
+++ b/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb.tt
diff --git a/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb.tt
index 481c70201b..481c70201b 100644
--- a/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
+++ b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb.tt
diff --git a/activerecord/lib/rails/generators/active_record/model/templates/model.rb b/activerecord/lib/rails/generators/active_record/model/templates/model.rb.tt
index 55dc65c8ad..55dc65c8ad 100644
--- a/activerecord/lib/rails/generators/active_record/model/templates/model.rb
+++ b/activerecord/lib/rails/generators/active_record/model/templates/model.rb.tt
diff --git a/activerecord/lib/rails/generators/active_record/model/templates/module.rb b/activerecord/lib/rails/generators/active_record/model/templates/module.rb.tt
index a3bf1c37b6..a3bf1c37b6 100644
--- a/activerecord/lib/rails/generators/active_record/model/templates/module.rb
+++ b/activerecord/lib/rails/generators/active_record/model/templates/module.rb.tt