Move Attribute and AttributeSet to ActiveModel

Use these to back the attributes API.  Stop automatically including
ActiveModel::Dirty in ActiveModel::Attributes, and make it optional.

10 files changed, 828 insertions, 74 deletions

diff --git a/activemodel/lib/active_model.rb b/activemodel/lib/active_model.rb
index a97d7989be..faecd6b96f 100644
--- a/activemodel/lib/active_model.rb
+++ b/activemodel/lib/active_model.rb
@@ -30,6 +30,8 @@ require "active_model/version"
 module ActiveModel
   extend ActiveSupport::Autoload
 
+  autoload :Attribute
+  autoload :Attributes
   autoload :AttributeAssignment
   autoload :AttributeMethods
   autoload :BlockValidator, "active_model/validator"
diff --git a/activemodel/lib/active_model/attribute.rb b/activemodel/lib/active_model/attribute.rb
new file mode 100644
index 0000000000..43130c37c5
--- /dev/null
+++ b/activemodel/lib/active_model/attribute.rb
@@ -0,0 +1,241 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  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
+
+    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/activemodel/lib/active_model/attribute/user_provided_default.rb b/activemodel/lib/active_model/attribute/user_provided_default.rb
new file mode 100644
index 0000000000..f274b687d4
--- /dev/null
+++ b/activemodel/lib/active_model/attribute/user_provided_default.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require "active_model/attribute"
+
+module ActiveModel
+  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
+
+      protected
+
+        attr_reader :user_provided_value
+    end
+  end
+end
diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
new file mode 100644
index 0000000000..9072460124
--- /dev/null
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -0,0 +1,114 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  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
+
+    def force_change(*)
+    end
+  end
+end
diff --git a/activemodel/lib/active_model/attribute_set.rb b/activemodel/lib/active_model/attribute_set.rb
new file mode 100644
index 0000000000..a892accbc6
--- /dev/null
+++ b/activemodel/lib/active_model/attribute_set.rb
@@ -0,0 +1,113 @@
+# frozen_string_literal: true
+
+require "active_model/attribute_set/builder"
+require "active_model/attribute_set/yaml_encoder"
+
+module ActiveModel
+  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/activemodel/lib/active_model/attribute_set/builder.rb b/activemodel/lib/active_model/attribute_set/builder.rb
new file mode 100644
index 0000000000..f94f47370f
--- /dev/null
+++ b/activemodel/lib/active_model/attribute_set/builder.rb
@@ -0,0 +1,124 @@
+# frozen_string_literal: true
+
+require "active_model/attribute"
+
+module ActiveModel
+  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
+
+    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/activemodel/lib/active_model/attribute_set/yaml_encoder.rb b/activemodel/lib/active_model/attribute_set/yaml_encoder.rb
new file mode 100644
index 0000000000..4ea945b956
--- /dev/null
+++ b/activemodel/lib/active_model/attribute_set/yaml_encoder.rb
@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  class AttributeSet
+    # Attempts to do more intelligent YAML dumping of an
+    # ActiveModel::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
+
+      protected
+
+        attr_reader :default_types
+    end
+  end
+end
diff --git a/activemodel/lib/active_model/attributes.rb b/activemodel/lib/active_model/attributes.rb
index 3e34d3b83a..13cad87875 100644
--- a/activemodel/lib/active_model/attributes.rb
+++ b/activemodel/lib/active_model/attributes.rb
@@ -1,24 +1,30 @@
 # frozen_string_literal: true
 
+require "active_support/core_ext/object/deep_dup"
 require "active_model/type"
+require "active_model/attribute_set"
+require "active_model/attribute/user_provided_default"
 
 module ActiveModel
   module Attributes #:nodoc:
     extend ActiveSupport::Concern
     include ActiveModel::AttributeMethods
-    include ActiveModel::Dirty
 
     included do
       attribute_method_suffix "="
       class_attribute :attribute_types, :_default_attributes, instance_accessor: false
-      self.attribute_types = {}
-      self._default_attributes = {}
+      self.attribute_types = Hash.new(Type.default_value)
+      self._default_attributes = AttributeSet.new({})
     end
 
     module ClassMethods
-      def attribute(name, cast_type = Type::Value.new, **options)
-        self.attribute_types = attribute_types.merge(name.to_s => cast_type)
-        self._default_attributes = _default_attributes.merge(name.to_s => options[:default])
+      def attribute(name, type = Type::Value.new, **options)
+        name = name.to_s
+        if type.is_a?(Symbol)
+          type = ActiveModel::Type.lookup(type, **options.except(:default))
+        end
+        self.attribute_types = attribute_types.merge(name => type)
+        define_default_attribute(name, options.fetch(:default, NO_DEFAULT_PROVIDED), type)
         define_attribute_methods(name)
       end
 
@@ -37,11 +43,29 @@ module ActiveModel
             undef_method :__temp__#{safe_name}=
           STR
         end
+
+        NO_DEFAULT_PROVIDED = Object.new # :nodoc:
+        private_constant :NO_DEFAULT_PROVIDED
+
+        def define_default_attribute(name, value, type)
+          self._default_attributes = _default_attributes.deep_dup
+          if value == NO_DEFAULT_PROVIDED
+            default_attribute = _default_attributes[name].with_type(type)
+          else
+            default_attribute = Attribute::UserProvidedDefault.new(
+              name,
+              value,
+              type,
+              _default_attributes.fetch(name.to_s) { nil },
+            )
+          end
+          _default_attributes[name] = default_attribute
+        end
     end
 
     def initialize(*)
+      @attributes = self.class._default_attributes.deep_dup
       super
-      clear_changes_information
     end
 
     private
@@ -53,21 +77,17 @@ module ActiveModel
           attr_name.to_s
         end
 
-        cast_type = self.class.attribute_types[name]
-
-        deserialized_value = ActiveModel::Type.lookup(cast_type).cast(value)
-        attribute_will_change!(name) unless deserialized_value == attribute(name)
-        instance_variable_set("@#{name}", deserialized_value)
-        deserialized_value
+        @attributes.write_from_user(attr_name.to_s, value)
+        value
       end
 
-      def attribute(name)
-        if instance_variable_defined?("@#{name}")
-          instance_variable_get("@#{name}")
+      def attribute(attr_name)
+        name = if self.class.attribute_alias?(attr_name)
+          self.class.attribute_alias(attr_name).to_s
         else
-          default = self.class._default_attributes[name]
-          default.respond_to?(:call) ? default.call : default
+          attr_name.to_s
         end
+        @attributes.fetch_value(name)
       end
 
       # Handle *= for method_missing.
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index 943db0ab52..ddd93e34a6 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -2,6 +2,7 @@
 
 require "active_support/hash_with_indifferent_access"
 require "active_support/core_ext/object/duplicable"
+require "active_model/attribute_mutation_tracker"
 
 module ActiveModel
   # == Active \Model \Dirty
@@ -130,6 +131,24 @@ module ActiveModel
       attribute_method_affix prefix: "restore_", suffix: "!"
     end
 
+    def initialize_dup(other) # :nodoc:
+      super
+      if self.class.respond_to?(:_default_attributes)
+        @attributes = self.class._default_attributes.map do |attr|
+          attr.with_value_from_user(@attributes.fetch_value(attr.name))
+        end
+      end
+      @mutations_from_database = nil
+    end
+
+    def changes_applied # :nodoc:
+      @previously_changed = changes
+      @mutations_before_last_save = mutations_from_database
+      @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
+      forget_attribute_assignments
+      @mutations_from_database = nil
+    end
+
     # Returns +true+ if any of the attributes have unsaved changes, +false+ otherwise.
     #
     #   person.changed? # => false
@@ -148,6 +167,60 @@ module ActiveModel
       changed_attributes.keys
     end
 
+    # Handles <tt>*_changed?</tt> for +method_missing+.
+    def attribute_changed?(attr, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN) # :nodoc:
+      !!changes_include?(attr) &&
+        (to == OPTION_NOT_GIVEN || to == _read_attribute(attr)) &&
+        (from == OPTION_NOT_GIVEN || from == changed_attributes[attr])
+    end
+
+    # Handles <tt>*_was</tt> for +method_missing+.
+    def attribute_was(attr) # :nodoc:
+      attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr)
+    end
+
+    # Handles <tt>*_previously_changed?</tt> for +method_missing+.
+    def attribute_previously_changed?(attr) #:nodoc:
+      previous_changes_include?(attr)
+    end
+
+    # Restore all previous data of the provided attributes.
+    def restore_attributes(attributes = changed)
+      attributes.each { |attr| restore_attribute! attr }
+    end
+
+    # Clears all dirty data: current changes and previous changes.
+    def clear_changes_information
+      @previously_changed = ActiveSupport::HashWithIndifferentAccess.new
+      @mutations_before_last_save = nil
+      @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
+      forget_attribute_assignments
+      @mutations_from_database = nil
+    end
+
+    def clear_attribute_changes(attr_names)
+      attributes_changed_by_setter.except!(*attr_names)
+      attr_names.each do |attr_name|
+        clear_attribute_change(attr_name)
+      end
+    end
+
+    # Returns a hash of the attributes with unsaved changes indicating their original
+    # values like <tt>attr => original value</tt>.
+    #
+    #   person.name # => "bob"
+    #   person.name = 'robert'
+    #   person.changed_attributes # => {"name" => "bob"}
+    def changed_attributes
+      # 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
+        attributes_changed_by_setter.reverse_merge(mutations_from_database.changed_values).freeze
+      end
+    end
+
     # Returns a hash of changed attributes indicating their original
     # and new values like <tt>attr => [original value, new value]</tt>.
     #
@@ -155,7 +228,9 @@ module ActiveModel
     #   person.name = 'bob'
     #   person.changes # => { "name" => ["bill", "bob"] }
     def changes
-      ActiveSupport::HashWithIndifferentAccess[changed.map { |attr| [attr, attribute_change(attr)] }]
+      cache_changed_attributes do
+        ActiveSupport::HashWithIndifferentAccess[changed.map { |attr| [attr, attribute_change(attr)] }]
+      end
     end
 
     # Returns a hash of attributes that were changed before the model was saved.
@@ -166,45 +241,51 @@ module ActiveModel
     #   person.previous_changes # => {"name" => ["bob", "robert"]}
     def previous_changes
       @previously_changed ||= ActiveSupport::HashWithIndifferentAccess.new
+      @previously_changed.merge(mutations_before_last_save.changes)
     end
 
-    # Returns a hash of the attributes with unsaved changes indicating their original
-    # values like <tt>attr => original value</tt>.
-    #
-    #   person.name # => "bob"
-    #   person.name = 'robert'
-    #   person.changed_attributes # => {"name" => "bob"}
-    def changed_attributes
-      @changed_attributes ||= ActiveSupport::HashWithIndifferentAccess.new
+    def attribute_changed_in_place?(attr_name) # :nodoc:
+      mutations_from_database.changed_in_place?(attr_name)
     end
 
-    # Handles <tt>*_changed?</tt> for +method_missing+.
-    def attribute_changed?(attr, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN) # :nodoc:
-      !!changes_include?(attr) &&
-        (to == OPTION_NOT_GIVEN || to == _read_attribute(attr)) &&
-        (from == OPTION_NOT_GIVEN || from == changed_attributes[attr])
-    end
+    private
+      def clear_attribute_change(attr_name)
+        mutations_from_database.forget_change(attr_name)
+      end
 
-    # Handles <tt>*_was</tt> for +method_missing+.
-    def attribute_was(attr) # :nodoc:
-      attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr)
-    end
+      def mutations_from_database
+        unless defined?(@mutations_from_database)
+          @mutations_from_database = nil
+        end
+        @mutations_from_database ||= if @attributes
+          ActiveModel::AttributeMutationTracker.new(@attributes)
+        else
+          NullMutationTracker.instance
+        end
+      end
 
-    # Handles <tt>*_previously_changed?</tt> for +method_missing+.
-    def attribute_previously_changed?(attr) #:nodoc:
-      previous_changes_include?(attr)
-    end
+      def forget_attribute_assignments
+        @attributes = @attributes.map(&:forgetting_assignment) if @attributes
+      end
 
-    # Restore all previous data of the provided attributes.
-    def restore_attributes(attributes = changed)
-      attributes.each { |attr| restore_attribute! attr }
-    end
+      def mutations_before_last_save
+        @mutations_before_last_save ||= ActiveModel::NullMutationTracker.instance
+      end
 
-    private
+      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
 
       # Returns +true+ if attr_name is changed, +false+ otherwise.
       def changes_include?(attr_name)
-        attributes_changed_by_setter.include?(attr_name)
+        attributes_changed_by_setter.include?(attr_name) || mutations_from_database.changed?(attr_name)
       end
       alias attribute_changed_by_setter? changes_include?
 
@@ -214,18 +295,6 @@ module ActiveModel
         previous_changes.include?(attr_name)
       end
 
-      # Removes current changes and makes them accessible through +previous_changes+.
-      def changes_applied # :doc:
-        @previously_changed = changes
-        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
-      end
-
-      # Clears all dirty data: current changes and previous changes.
-      def clear_changes_information # :doc:
-        @previously_changed = ActiveSupport::HashWithIndifferentAccess.new
-        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
-      end
-
       # Handles <tt>*_change</tt> for +method_missing+.
       def attribute_change(attr)
         [changed_attributes[attr], _read_attribute(attr)] if attribute_changed?(attr)
@@ -238,15 +307,16 @@ module ActiveModel
 
       # Handles <tt>*_will_change!</tt> for +method_missing+.
       def attribute_will_change!(attr)
-        return if attribute_changed?(attr)
+        unless attribute_changed?(attr)
+          begin
+            value = _read_attribute(attr)
+            value = value.duplicable? ? value.clone : value
+          rescue TypeError, NoMethodError
+          end
 
-        begin
-          value = _read_attribute(attr)
-          value = value.duplicable? ? value.clone : value
-        rescue TypeError, NoMethodError
+          set_attribute_was(attr, value)
         end
-
-        set_attribute_was(attr, value)
+        mutations_from_database.force_change(attr)
       end
 
       # Handles <tt>restore_*!</tt> for +method_missing+.
@@ -257,18 +327,13 @@ module ActiveModel
         end
       end
 
-      # This is necessary because `changed_attributes` might be overridden in
-      # other implementations (e.g. in `ActiveRecord`)
-      alias_method :attributes_changed_by_setter, :changed_attributes # :nodoc:
+      def attributes_changed_by_setter
+        @attributes_changed_by_setter ||= ActiveSupport::HashWithIndifferentAccess.new
+      end
 
       # Force an attribute to have a particular "before" value
       def set_attribute_was(attr, old_value)
         attributes_changed_by_setter[attr] = old_value
       end
-
-      # Remove changes information for the provided attributes.
-      def clear_attribute_changes(attributes) # :doc:
-        attributes_changed_by_setter.except!(*attributes)
-      end
   end
 end
diff --git a/activemodel/lib/active_model/type.rb b/activemodel/lib/active_model/type.rb
index b0ed67f28e..39324999c9 100644
--- a/activemodel/lib/active_model/type.rb
+++ b/activemodel/lib/active_model/type.rb
@@ -32,6 +32,10 @@ module ActiveModel
       def lookup(*args, **kwargs) # :nodoc:
         registry.lookup(*args, **kwargs)
       end
+
+      def default_value # :nodoc:
+        @default_value ||= Value.new
+      end
     end
 
     register(:big_integer, Type::BigInteger)