Merge pull request #32313 from lulalala/model_error_as_object

Model error as object

14 files changed, 899 insertions, 182 deletions

diff --git a/activemodel/lib/active_model/error.rb b/activemodel/lib/active_model/error.rb
new file mode 100644
index 0000000000..5a1298e27f
--- /dev/null
+++ b/activemodel/lib/active_model/error.rb
@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  # == Active \Model \Error
+  #
+  # Represents one single error
+  class Error
+    CALLBACKS_OPTIONS = [:if, :unless, :on, :allow_nil, :allow_blank, :strict]
+    MESSAGE_OPTIONS = [:message]
+
+    def initialize(base, attribute, type = :invalid, **options)
+      @base = base
+      @attribute = attribute
+      @raw_type = type
+      @type = type || :invalid
+      @options = options
+    end
+
+    def initialize_dup(other)
+      @attribute = @attribute.dup
+      @raw_type = @raw_type.dup
+      @type = @type.dup
+      @options = @options.deep_dup
+    end
+
+    attr_reader :base, :attribute, :type, :raw_type, :options
+
+    def message
+      case raw_type
+      when Symbol
+        base.errors.generate_message(attribute, raw_type, options.except(*CALLBACKS_OPTIONS))
+      else
+        raw_type
+      end
+    end
+
+    def detail
+      { error: raw_type }.merge(options.except(*CALLBACKS_OPTIONS + MESSAGE_OPTIONS))
+    end
+
+    def full_message
+      base.errors.full_message(attribute, message)
+    end
+
+    # See if error matches provided +attribute+, +type+ and +options+.
+    def match?(attribute, type = nil, **options)
+      if @attribute != attribute || (type && @type != type)
+        return false
+      end
+
+      options.each do |key, value|
+        if @options[key] != value
+          return false
+        end
+      end
+
+      true
+    end
+
+    def strict_match?(attribute, type, **options)
+      return false unless match?(attribute, type, **options)
+
+      full_message == Error.new(@base, attribute, type, **options).full_message
+    end
+
+    def ==(other)
+      other.is_a?(self.class) && attributes_for_hash == other.attributes_for_hash
+    end
+    alias eql? ==
+
+    def hash
+      attributes_for_hash.hash
+    end
+
+    protected
+
+      def attributes_for_hash
+        [@base, @attribute, @raw_type, @options]
+      end
+  end
+end
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 3a692a3e64..a9af426fb1 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -4,6 +4,10 @@ require "active_support/core_ext/array/conversions"
 require "active_support/core_ext/string/inflections"
 require "active_support/core_ext/object/deep_dup"
 require "active_support/core_ext/string/filters"
+require "active_support/deprecation"
+require "active_model/error"
+require "active_model/nested_error"
+require "forwardable"
 
 module ActiveModel
   # == Active \Model \Errors
@@ -59,15 +63,20 @@ module ActiveModel
   class Errors
     include Enumerable
 
-    CALLBACKS_OPTIONS = [:if, :unless, :on, :allow_nil, :allow_blank, :strict]
-    MESSAGE_OPTIONS = [:message]
+    extend Forwardable
+    def_delegators :@errors, :size, :clear, :blank?, :empty?, :uniq!
+    # TODO: forward all enumerable methods after `each` deprecation is removed.
+    def_delegators :@errors, :count
+
+    LEGACY_ATTRIBUTES = [:messages, :details].freeze
 
     class << self
       attr_accessor :i18n_customize_full_message # :nodoc:
     end
     self.i18n_customize_full_message = false
 
-    attr_reader :messages, :details
+    attr_reader :errors
+    alias :objects :errors
 
     # Pass in the instance of the object that is using the errors object.
     #
@@ -77,18 +86,17 @@ module ActiveModel
     #     end
     #   end
     def initialize(base)
-      @base     = base
-      @messages = apply_default_array({})
-      @details = apply_default_array({})
+      @base = base
+      @errors = []
     end
 
     def initialize_dup(other) # :nodoc:
-      @messages = other.messages.dup
-      @details  = other.details.deep_dup
+      @errors = other.errors.deep_dup
       super
     end
 
     # Copies the errors from <tt>other</tt>.
+    # For copying errors but keep <tt>@base</tt> as is.
     #
     # other - The ActiveModel::Errors instance.
     #
@@ -96,11 +104,31 @@ module ActiveModel
     #
     #   person.errors.copy!(other)
     def copy!(other) # :nodoc:
-      @messages = other.messages.dup
-      @details  = other.details.dup
+      @errors = other.errors.deep_dup
+      @errors.each { |error|
+        error.instance_variable_set("@base", @base)
+      }
+    end
+
+    # Imports one error
+    # Imported errors are wrapped as a NestedError,
+    # providing access to original error object.
+    # If attribute or type needs to be overriden, use `override_options`.
+    #
+    # override_options - Hash
+    # @option override_options [Symbol] :attribute Override the attribute the error belongs to
+    # @option override_options [Symbol] :type Override type of the error.
+    def import(error, override_options = {})
+      [:attribute, :type].each do |key|
+        if override_options.key?(key)
+          override_options[key] = override_options[key].to_sym
+        end
+      end
+      @errors.append(NestedError.new(@base, error, override_options))
     end
 
-    # Merges the errors from <tt>other</tt>.
+    # Merges the errors from <tt>other</tt>,
+    # each <tt>Error</tt> wrapped as <tt>NestedError</tt>.
     #
     # other - The ActiveModel::Errors instance.
     #
@@ -108,8 +136,9 @@ module ActiveModel
     #
     #   person.errors.merge!(other)
     def merge!(other)
-      @messages.merge!(other.messages) { |_, ary1, ary2| ary1 + ary2 }
-      @details.merge!(other.details) { |_, ary1, ary2| ary1 + ary2 }
+      other.errors.each { |error|
+        import(error)
+      }
     end
 
     # Removes all errors except the given keys. Returns a hash containing the removed errors.
@@ -118,19 +147,31 @@ module ActiveModel
     #   person.errors.slice!(:age, :gender) # => { :name=>["cannot be nil"], :city=>["cannot be nil"] }
     #   person.errors.keys                  # => [:age, :gender]
     def slice!(*keys)
+      deprecation_removal_warning(:slice!)
+
       keys = keys.map(&:to_sym)
-      @details.slice!(*keys)
-      @messages.slice!(*keys)
+
+      results = messages.dup.slice!(*keys)
+
+      @errors.keep_if do |error|
+        keys.include?(error.attribute)
+      end
+
+      results
     end
 
-    # Clear the error messages.
+    # Search for errors matching +attribute+, +type+ or +options+.
+    #
+    # Only supplied params will be matched.
     #
-    #   person.errors.full_messages # => ["name cannot be nil"]
-    #   person.errors.clear
-    #   person.errors.full_messages # => []
-    def clear
-      messages.clear
-      details.clear
+    #   person.errors.where(:name) # => all name errors.
+    #   person.errors.where(:name, :too_short) # => all name errors being too short
+    #   person.errors.where(:name, :too_short, minimum: 2) # => all name errors being too short and minimum is 2
+    def where(attribute, type = nil, **options)
+      attribute, type, options = normalize_arguments(attribute, type, options)
+      @errors.select { |error|
+        error.match?(attribute, type, options)
+      }
     end
 
     # Returns +true+ if the error messages include an error for the given key
@@ -140,8 +181,9 @@ module ActiveModel
     #   person.errors.include?(:name) # => true
     #   person.errors.include?(:age)  # => false
     def include?(attribute)
-      attribute = attribute.to_sym
-      messages.key?(attribute) && messages[attribute].present?
+      @errors.any? { |error|
+        error.match?(attribute.to_sym)
+      }
     end
     alias :has_key? :include?
     alias :key? :include?
@@ -151,10 +193,13 @@ module ActiveModel
     #   person.errors[:name]        # => ["cannot be nil"]
     #   person.errors.delete(:name) # => ["cannot be nil"]
     #   person.errors[:name]        # => []
-    def delete(key)
-      attribute = key.to_sym
-      details.delete(attribute)
-      messages.delete(attribute)
+    def delete(attribute, type = nil, **options)
+      attribute, type, options = normalize_arguments(attribute, type, options)
+      matches = where(attribute, type, options)
+      matches.each do |error|
+        @errors.delete(error)
+      end
+      matches.map(&:message)
     end
 
     # When passed a symbol or a name of a method, returns an array of errors
@@ -163,7 +208,7 @@ module ActiveModel
     #   person.errors[:name]  # => ["cannot be nil"]
     #   person.errors['name'] # => ["cannot be nil"]
     def [](attribute)
-      messages[attribute.to_sym]
+      DeprecationHandlingMessageArray.new(messages_for(attribute), self, attribute)
     end
 
     # Iterates through each error key, value pair in the error messages hash.
@@ -180,31 +225,37 @@ module ActiveModel
     #     # Will yield :name and "can't be blank"
     #     # then yield :name and "must be specified"
     #   end
-    def each
-      messages.each_key do |attribute|
-        messages[attribute].each { |error| yield attribute, error }
-      end
-    end
+    def each(&block)
+      if block.arity == 1
+        @errors.each(&block)
+      else
+        ActiveSupport::Deprecation.warn(<<-MSG.squish)
+          Enumerating ActiveModel::Errors as a hash has been deprecated.
+          In Rails 6, `errors` is an array of Error objects,
+          therefore it should be accessed by a block with a single block
+          parameter like this:
+
+          person.errors.each do |error|
+            error.full_message
+          end
 
-    # Returns the number of error messages.
-    #
-    #   person.errors.add(:name, :blank, message: "can't be blank")
-    #   person.errors.size # => 1
-    #   person.errors.add(:name, :not_specified, message: "must be specified")
-    #   person.errors.size # => 2
-    def size
-      values.flatten.size
+          You are passing a block expecting 2 parameters,
+          so the old hash behavior is simulated. As this is deprecated,
+          this will result in an ArgumentError in Rails 6.1.
+        MSG
+        @errors.
+          sort { |a, b| a.attribute <=> b.attribute }.
+          each { |error| yield error.attribute, error.message }
+      end
     end
-    alias :count :size
 
     # Returns all message values.
     #
     #   person.errors.messages # => {:name=>["cannot be nil", "must be specified"]}
     #   person.errors.values   # => [["cannot be nil", "must be specified"]]
     def values
-      messages.select do |key, value|
-        !value.empty?
-      end.values
+      deprecation_removal_warning(:values)
+      @errors.map(&:message).freeze
     end
 
     # Returns all message keys.
@@ -212,21 +263,12 @@ module ActiveModel
     #   person.errors.messages # => {:name=>["cannot be nil", "must be specified"]}
     #   person.errors.keys     # => [:name]
     def keys
-      messages.select do |key, value|
-        !value.empty?
-      end.keys
+      deprecation_removal_warning(:keys)
+      keys = @errors.map(&:attribute)
+      keys.uniq!
+      keys.freeze
     end
 
-    # Returns +true+ if no errors are found, +false+ otherwise.
-    # If the error message is a string it can be empty.
-    #
-    #   person.errors.full_messages # => ["name cannot be nil"]
-    #   person.errors.empty?        # => false
-    def empty?
-      size.zero?
-    end
-    alias :blank? :empty?
-
     # Returns an xml formatted representation of the Errors hash.
     #
     #   person.errors.add(:name, :blank, message: "can't be blank")
@@ -239,6 +281,7 @@ module ActiveModel
     #   #    <error>name must be specified</error>
     #   #  </errors>
     def to_xml(options = {})
+      deprecation_removal_warning(:to_xml)
       to_a.to_xml({ root: "errors", skip_types: true }.merge!(options))
     end
 
@@ -258,13 +301,28 @@ module ActiveModel
     #   person.errors.to_hash       # => {:name=>["cannot be nil"]}
     #   person.errors.to_hash(true) # => {:name=>["name cannot be nil"]}
     def to_hash(full_messages = false)
-      if full_messages
-        messages.each_with_object({}) do |(attribute, array), messages|
-          messages[attribute] = array.map { |message| full_message(attribute, message) }
-        end
-      else
-        without_default_proc(messages)
+      hash = {}
+      message_method = full_messages ? :full_message : :message
+      group_by_attribute.each do |attribute, errors|
+        hash[attribute] = errors.map(&message_method)
       end
+      hash
+    end
+
+    def messages
+      DeprecationHandlingMessageHash.new(self)
+    end
+
+    def details
+      hash = {}
+      group_by_attribute.each do |attribute, errors|
+        hash[attribute] = errors.map(&:detail)
+      end
+      DeprecationHandlingDetailsHash.new(hash)
+    end
+
+    def group_by_attribute
+      @errors.group_by(&:attribute)
     end
 
     # Adds +message+ to the error messages and used validator type to +details+ on +attribute+.
@@ -308,17 +366,20 @@ module ActiveModel
     #   # => {:base=>["either name or email must be present"]}
     #   person.errors.details
     #   # => {:base=>[{error: :name_or_email_blank}]}
-    def add(attribute, message = :invalid, options = {})
-      message = message.call if message.respond_to?(:call)
-      detail  = normalize_detail(message, options)
-      message = normalize_message(attribute, message, options)
+    def add(attribute, type = :invalid, **options)
+      error = Error.new(
+        @base,
+        *normalize_arguments(attribute, type, options)
+      )
+
       if exception = options[:strict]
         exception = ActiveModel::StrictValidationFailed if exception == true
-        raise exception, full_message(attribute, message)
+        raise exception, error.full_message
       end
 
-      details[attribute.to_sym]  << detail
-      messages[attribute.to_sym] << message
+      @errors.append(error)
+
+      error
     end
 
     # Returns +true+ if an error on the attribute with the given message is
@@ -337,13 +398,15 @@ module ActiveModel
     #   person.errors.added? :name, :too_long, count: 24                     # => false
     #   person.errors.added? :name, :too_long                                # => false
     #   person.errors.added? :name, "is too long"                            # => false
-    def added?(attribute, message = :invalid, options = {})
-      message = message.call if message.respond_to?(:call)
+    def added?(attribute, type = :invalid, options = {})
+      attribute, type, options = normalize_arguments(attribute, type, options)
 
-      if message.is_a? Symbol
-        details[attribute.to_sym].include? normalize_detail(message, options)
+      if type.is_a? Symbol
+        @errors.any? { |error|
+          error.strict_match?(attribute, type, options)
+        }
       else
-        self[attribute].include? message
+        messages_for(attribute).include?(type)
       end
     end
 
@@ -359,12 +422,12 @@ module ActiveModel
     #   person.errors.of_kind? :name, :not_too_long                            # => false
     #   person.errors.of_kind? :name, "is too long"                            # => false
     def of_kind?(attribute, message = :invalid)
-      message = message.call if message.respond_to?(:call)
+      attribute, message = normalize_arguments(attribute, message)
 
       if message.is_a? Symbol
-        details[attribute.to_sym].map { |e| e[:error] }.include? message
+        !where(attribute, message).empty?
       else
-        self[attribute].include? message
+        messages_for(attribute).include?(message)
       end
     end
 
@@ -379,7 +442,7 @@ module ActiveModel
     #   person.errors.full_messages
     #   # => ["Name is too short (minimum is 5 characters)", "Name can't be blank", "Email can't be blank"]
     def full_messages
-      map { |attribute, message| full_message(attribute, message) }
+      @errors.map(&:full_message)
     end
     alias :to_a :full_messages
 
@@ -394,21 +457,16 @@ module ActiveModel
     #   person.errors.full_messages_for(:name)
     #   # => ["Name is too short (minimum is 5 characters)", "Name can't be blank"]
     def full_messages_for(attribute)
-      attribute = attribute.to_sym
-      messages[attribute].map { |message| full_message(attribute, message) }
+      where(attribute).map(&:full_message).freeze
+    end
+
+    def messages_for(attribute)
+      where(attribute).map(&:message)
     end
 
     # Returns a full message for a given attribute.
     #
     #   person.errors.full_message(:name, 'is invalid') # => "Name is invalid"
-    #
-    # The `"%{attribute} %{message}"` error format can be overridden with either
-    #
-    # * <tt>activemodel.errors.models.person/contacts/addresses.attributes.street.format</tt>
-    # * <tt>activemodel.errors.models.person/contacts/addresses.format</tt>
-    # * <tt>activemodel.errors.models.person.attributes.name.format</tt>
-    # * <tt>activemodel.errors.models.person.format</tt>
-    # * <tt>errors.format</tt>
     def full_message(attribute, message)
       return message if attribute == :base
       attribute = attribute.to_s
@@ -514,46 +572,111 @@ module ActiveModel
       I18n.translate(key, options)
     end
 
-    def marshal_dump # :nodoc:
-      [@base, without_default_proc(@messages), without_default_proc(@details)]
-    end
-
     def marshal_load(array) # :nodoc:
-      @base, @messages, @details = array
-      apply_default_array(@messages)
-      apply_default_array(@details)
+      # Rails 5
+      @errors = []
+      @base = array[0]
+      add_from_legacy_details_hash(array[2])
     end
 
     def init_with(coder) # :nodoc:
-      coder.map.each { |k, v| instance_variable_set(:"@#{k}", v) }
-      @details ||= {}
-      apply_default_array(@messages)
-      apply_default_array(@details)
+      data = coder.map
+
+      data.each { |k, v|
+        next if LEGACY_ATTRIBUTES.include?(k.to_sym)
+        instance_variable_set(:"@#{k}", v)
+      }
+
+      @errors ||= []
+
+      # Legacy support Rails 5.x details hash
+      add_from_legacy_details_hash(data["details"]) if data.key?("details")
     end
 
-  private
-    def normalize_message(attribute, message, options)
-      case message
-      when Symbol
-        generate_message(attribute, message, options.except(*CALLBACKS_OPTIONS))
-      else
-        message
+    private
+
+      def normalize_arguments(attribute, type, **options)
+        # Evaluate proc first
+        if type.respond_to?(:call)
+          type = type.call(@base, options)
+        end
+
+        [attribute.to_sym, type, options]
+      end
+
+      def add_from_legacy_details_hash(details)
+        details.each { |attribute, errors|
+          errors.each { |error|
+            type = error.delete(:error)
+            add(attribute, type, error)
+          }
+        }
+      end
+
+      def deprecation_removal_warning(method_name)
+        ActiveSupport::Deprecation.warn("ActiveModel::Errors##{method_name} is deprecated and will be removed in Rails 6.1")
+      end
+
+      def deprecation_rename_warning(old_method_name, new_method_name)
+        ActiveSupport::Deprecation.warn("ActiveModel::Errors##{old_method_name} is deprecated. Please call ##{new_method_name} instead.")
       end
+  end
+
+  class DeprecationHandlingMessageHash < SimpleDelegator
+    def initialize(errors)
+      @errors = errors
+      super(prepare_content)
     end
 
-    def normalize_detail(message, options)
-      { error: message }.merge(options.except(*CALLBACKS_OPTIONS + MESSAGE_OPTIONS))
+    def []=(attribute, value)
+      ActiveSupport::Deprecation.warn("Calling `[]=` to an ActiveModel::Errors is deprecated. Please call `ActiveModel::Errors#add` instead.")
+
+      @errors.delete(attribute)
+      Array(value).each do |message|
+        @errors.add(attribute, message)
+      end
+
+      __setobj__ prepare_content
     end
 
-    def without_default_proc(hash)
-      hash.dup.tap do |new_h|
-        new_h.default_proc = nil
+    private
+
+      def prepare_content
+        content = @errors.to_hash
+        content.each do |attribute, value|
+          content[attribute] = DeprecationHandlingMessageArray.new(value, @errors, attribute)
+        end
+        content.default_proc = proc do |hash, attribute|
+          hash = hash.dup
+          hash[attribute] = DeprecationHandlingMessageArray.new([], @errors, attribute)
+          __setobj__ hash.freeze
+          hash[attribute]
+        end
+        content.freeze
       end
+  end
+
+  class DeprecationHandlingMessageArray < SimpleDelegator
+    def initialize(content, errors, attribute)
+      @errors = errors
+      @attribute = attribute
+      super(content.freeze)
     end
 
-    def apply_default_array(hash)
-      hash.default_proc = proc { |h, key| h[key] = [] }
-      hash
+    def <<(message)
+      ActiveSupport::Deprecation.warn("Calling `<<` to an ActiveModel::Errors message array in order to add an error is deprecated. Please call `ActiveModel::Errors#add` instead.")
+
+      @errors.add(@attribute, message)
+      __setobj__ @errors.messages_for(@attribute)
+      self
+    end
+  end
+
+  class DeprecationHandlingDetailsHash < SimpleDelegator
+    def initialize(details)
+      details.default = []
+      details.freeze
+      super(details)
     end
   end
 
diff --git a/activemodel/lib/active_model/lint.rb b/activemodel/lib/active_model/lint.rb
index b7ceabb59a..f9bfed95f1 100644
--- a/activemodel/lib/active_model/lint.rb
+++ b/activemodel/lib/active_model/lint.rb
@@ -101,7 +101,7 @@ module ActiveModel
       # locale. If no error is present, the method should return an empty array.
       def test_errors_aref
         assert_respond_to model, :errors
-        assert model.errors[:hello].is_a?(Array), "errors#[] should return an Array"
+        assert_equal [], model.errors[:hello], "errors#[] should return an empty Array"
       end
 
       private
diff --git a/activemodel/lib/active_model/nested_error.rb b/activemodel/lib/active_model/nested_error.rb
new file mode 100644
index 0000000000..93348c7771
--- /dev/null
+++ b/activemodel/lib/active_model/nested_error.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require "active_model/error"
+require "forwardable"
+
+module ActiveModel
+  # Represents one single error
+  # @!attribute [r] base
+  #   @return [ActiveModel::Base] the object which the error belongs to
+  # @!attribute [r] attribute
+  #   @return [Symbol] attribute of the object which the error belongs to
+  # @!attribute [r] type
+  #   @return [Symbol] error's type
+  # @!attribute [r] options
+  #   @return [Hash] additional options
+  # @!attribute [r] inner_error
+  #   @return [Error] inner error
+  class NestedError < Error
+    def initialize(base, inner_error, override_options = {})
+      @base = base
+      @inner_error = inner_error
+      @attribute = override_options.fetch(:attribute) { inner_error.attribute }
+      @type = override_options.fetch(:type) { inner_error.type }
+      @raw_type = inner_error.raw_type
+      @options = inner_error.options
+    end
+
+    attr_reader :inner_error
+
+    extend Forwardable
+    def_delegators :@inner_error, :message
+  end
+end
diff --git a/activemodel/test/cases/error_test.rb b/activemodel/test/cases/error_test.rb
new file mode 100644
index 0000000000..d74321fee5
--- /dev/null
+++ b/activemodel/test/cases/error_test.rb
@@ -0,0 +1,200 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "active_model/error"
+
+class ErrorTest < ActiveModel::TestCase
+  class Person
+    extend ActiveModel::Naming
+    def initialize
+      @errors = ActiveModel::Errors.new(self)
+    end
+
+    attr_accessor :name, :age
+    attr_reader   :errors
+
+    def read_attribute_for_validation(attr)
+      send(attr)
+    end
+
+    def self.human_attribute_name(attr, options = {})
+      attr
+    end
+
+    def self.lookup_ancestors
+      [self]
+    end
+  end
+
+  def test_initialize
+    base = Person.new
+    error = ActiveModel::Error.new(base, :name, :too_long, foo: :bar)
+    assert_equal base, error.base
+    assert_equal :name, error.attribute
+    assert_equal :too_long, error.type
+    assert_equal({ foo: :bar }, error.options)
+  end
+
+  test "initialize without type" do
+    error = ActiveModel::Error.new(Person.new, :name)
+    assert_equal :invalid, error.type
+    assert_equal({}, error.options)
+  end
+
+  test "initialize without type but with options" do
+    options = { message: "bar" }
+    error = ActiveModel::Error.new(Person.new, :name, options)
+    assert_equal(options, error.options)
+  end
+
+  # match?
+
+  test "match? handles mixed condition" do
+    subject = ActiveModel::Error.new(Person.new, :mineral, :not_enough, count: 2)
+    assert_not subject.match?(:mineral, :too_coarse)
+    assert subject.match?(:mineral, :not_enough)
+    assert subject.match?(:mineral, :not_enough, count: 2)
+    assert_not subject.match?(:mineral, :not_enough, count: 1)
+  end
+
+  test "match? handles attribute match" do
+    subject = ActiveModel::Error.new(Person.new, :mineral, :not_enough, count: 2)
+    assert_not subject.match?(:foo)
+    assert subject.match?(:mineral)
+  end
+
+  test "match? handles error type match" do
+    subject = ActiveModel::Error.new(Person.new, :mineral, :not_enough, count: 2)
+    assert_not subject.match?(:mineral, :too_coarse)
+    assert subject.match?(:mineral, :not_enough)
+  end
+
+  test "match? handles extra options match" do
+    subject = ActiveModel::Error.new(Person.new, :mineral, :not_enough, count: 2)
+    assert_not subject.match?(:mineral, :not_enough, count: 1)
+    assert subject.match?(:mineral, :not_enough, count: 2)
+  end
+
+  # message
+
+  test "message with type as a symbol" do
+    error = ActiveModel::Error.new(Person.new, :name, :blank)
+    assert_equal "can't be blank", error.message
+  end
+
+  test "message with custom interpolation" do
+    subject = ActiveModel::Error.new(Person.new, :name, :inclusion, message: "custom message %{value}", value: "name")
+    assert_equal "custom message name", subject.message
+  end
+
+  test "message returns plural interpolation" do
+    subject = ActiveModel::Error.new(Person.new, :name, :too_long, count: 10)
+    assert_equal "is too long (maximum is 10 characters)", subject.message
+  end
+
+  test "message returns singular interpolation" do
+    subject = ActiveModel::Error.new(Person.new, :name, :too_long, count: 1)
+    assert_equal "is too long (maximum is 1 character)", subject.message
+  end
+
+  test "message returns count interpolation" do
+    subject = ActiveModel::Error.new(Person.new, :name, :too_long, message: "custom message %{count}", count: 10)
+    assert_equal "custom message 10", subject.message
+  end
+
+  test "message handles lambda in messages and option values, and i18n interpolation" do
+    subject = ActiveModel::Error.new(Person.new, :name, :invalid,
+      foo: "foo",
+      bar: "bar",
+      baz: Proc.new { "baz" },
+      message: Proc.new { |model, options|
+        "%{attribute} %{foo} #{options[:bar]} %{baz}"
+      }
+    )
+    assert_equal "name foo bar baz", subject.message
+  end
+
+  test "generate_message works without i18n_scope" do
+    person = Person.new
+    error = ActiveModel::Error.new(person, :name, :blank)
+    assert_not_respond_to Person, :i18n_scope
+    assert_nothing_raised {
+      error.message
+    }
+  end
+
+  test "message with type as custom message" do
+    error = ActiveModel::Error.new(Person.new, :name, message: "cannot be blank")
+    assert_equal "cannot be blank", error.message
+  end
+
+  test "message with options[:message] as custom message" do
+    error = ActiveModel::Error.new(Person.new, :name, :blank, message: "cannot be blank")
+    assert_equal "cannot be blank", error.message
+  end
+
+  test "message renders lazily using current locale" do
+    error = nil
+
+    I18n.backend.store_translations(:pl, errors: { messages: { invalid: "jest nieprawidłowe" } })
+
+    I18n.with_locale(:en) { error = ActiveModel::Error.new(Person.new, :name, :invalid) }
+    I18n.with_locale(:pl) {
+      assert_equal "jest nieprawidłowe", error.message
+    }
+  end
+
+  test "message uses current locale" do
+    I18n.backend.store_translations(:en, errors: { messages: { inadequate: "Inadequate %{attribute} found!" } })
+    error = ActiveModel::Error.new(Person.new, :name, :inadequate)
+    assert_equal "Inadequate name found!", error.message
+  end
+
+  # full_message
+
+  test "full_message returns the given message when attribute is :base" do
+    error = ActiveModel::Error.new(Person.new, :base, message: "press the button")
+    assert_equal "press the button", error.full_message
+  end
+
+  test "full_message returns the given message with the attribute name included" do
+    error = ActiveModel::Error.new(Person.new, :name, :blank)
+    assert_equal "name can't be blank", error.full_message
+  end
+
+  test "full_message uses default format" do
+    error = ActiveModel::Error.new(Person.new, :name, message: "can't be blank")
+
+    # Use a locale without errors.format
+    I18n.with_locale(:unknown) {
+      assert_equal "name can't be blank", error.full_message
+    }
+  end
+
+  test "equality by base attribute, type and options" do
+    person = Person.new
+
+    e1 = ActiveModel::Error.new(person, :name, foo: :bar)
+    e2 = ActiveModel::Error.new(person, :name, foo: :bar)
+    e2.instance_variable_set(:@_humanized_attribute, "Name")
+
+    assert_equal(e1, e2)
+  end
+
+  test "inequality" do
+    person = Person.new
+    error = ActiveModel::Error.new(person, :name, foo: :bar)
+
+    assert error != ActiveModel::Error.new(person, :name, foo: :baz)
+    assert error != ActiveModel::Error.new(person, :name)
+    assert error != ActiveModel::Error.new(person, :title, foo: :bar)
+    assert error != ActiveModel::Error.new(Person.new, :name, foo: :bar)
+  end
+
+  test "comparing against different class would not raise error" do
+    person = Person.new
+    error = ActiveModel::Error.new(person, :name, foo: :bar)
+
+    assert error != person
+  end
+end
diff --git a/activemodel/test/cases/errors_test.rb b/activemodel/test/cases/errors_test.rb
index 947f9bf99b..0837e9db96 100644
--- a/activemodel/test/cases/errors_test.rb
+++ b/activemodel/test/cases/errors_test.rb
@@ -10,7 +10,7 @@ class ErrorsTest < ActiveModel::TestCase
       @errors = ActiveModel::Errors.new(self)
     end
 
-    attr_accessor :name, :age
+    attr_accessor :name, :age, :gender, :city
     attr_reader   :errors
 
     def validate!
@@ -31,48 +31,47 @@ class ErrorsTest < ActiveModel::TestCase
   end
 
   def test_delete
-    errors = ActiveModel::Errors.new(self)
-    errors[:foo] << "omg"
-    errors.delete("foo")
-    assert_empty errors[:foo]
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :blank)
+    errors.delete("name")
+    assert_empty errors[:name]
   end
 
   def test_include?
-    errors = ActiveModel::Errors.new(self)
-    errors[:foo] << "omg"
+    errors = ActiveModel::Errors.new(Person.new)
+    assert_deprecated { errors[:foo] << "omg" }
     assert_includes errors, :foo, "errors should include :foo"
     assert_includes errors, "foo", "errors should include 'foo' as :foo"
   end
 
   def test_dup
-    errors = ActiveModel::Errors.new(self)
-    errors[:foo] << "bar"
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name)
     errors_dup = errors.dup
-    errors_dup[:bar] << "omg"
-    assert_not_same errors_dup.messages, errors.messages
+    assert_not_same errors_dup.errors, errors.errors
   end
 
   def test_has_key?
-    errors = ActiveModel::Errors.new(self)
-    errors[:foo] << "omg"
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:foo, "omg")
     assert_equal true, errors.has_key?(:foo), "errors should have key :foo"
     assert_equal true, errors.has_key?("foo"), "errors should have key 'foo' as :foo"
   end
 
   def test_has_no_key
-    errors = ActiveModel::Errors.new(self)
+    errors = ActiveModel::Errors.new(Person.new)
     assert_equal false, errors.has_key?(:name), "errors should not have key :name"
   end
 
   def test_key?
-    errors = ActiveModel::Errors.new(self)
-    errors[:foo] << "omg"
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:foo, "omg")
     assert_equal true, errors.key?(:foo), "errors should have key :foo"
     assert_equal true, errors.key?("foo"), "errors should have key 'foo' as :foo"
   end
 
   def test_no_key
-    errors = ActiveModel::Errors.new(self)
+    errors = ActiveModel::Errors.new(Person.new)
     assert_equal false, errors.key?(:name), "errors should not have key :name"
   end
 
@@ -86,42 +85,58 @@ class ErrorsTest < ActiveModel::TestCase
   end
 
   test "error access is indifferent" do
-    errors = ActiveModel::Errors.new(self)
-    errors[:foo] << "omg"
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, "omg")
 
-    assert_equal ["omg"], errors["foo"]
+    assert_equal ["omg"], errors["name"]
   end
 
   test "values returns an array of messages" do
+    errors = ActiveModel::Errors.new(Person.new)
+    assert_deprecated { errors.messages[:foo] = "omg" }
+    assert_deprecated { errors.messages[:baz] = "zomg" }
+
+    assert_deprecated do
+      assert_equal ["omg", "zomg"], errors.values
+    end
+  end
+
+  test "[]= overrides values" do
     errors = ActiveModel::Errors.new(self)
-    errors.messages[:foo] = "omg"
-    errors.messages[:baz] = "zomg"
+    assert_deprecated { errors.messages[:foo] = "omg" }
+    assert_deprecated { errors.messages[:foo] = "zomg" }
 
-    assert_equal ["omg", "zomg"], errors.values
+    assert_equal ["zomg"], errors[:foo]
   end
 
   test "values returns an empty array after try to get a message only" do
-    errors = ActiveModel::Errors.new(self)
+    errors = ActiveModel::Errors.new(Person.new)
     errors.messages[:foo]
     errors.messages[:baz]
 
-    assert_equal [], errors.values
+    assert_deprecated do
+      assert_equal [], errors.values
+    end
   end
 
   test "keys returns the error keys" do
-    errors = ActiveModel::Errors.new(self)
-    errors.messages[:foo] << "omg"
-    errors.messages[:baz] << "zomg"
+    errors = ActiveModel::Errors.new(Person.new)
+    assert_deprecated { errors.messages[:foo] << "omg" }
+    assert_deprecated { errors.messages[:baz] << "zomg" }
 
-    assert_equal [:foo, :baz], errors.keys
+    assert_deprecated do
+      assert_equal [:foo, :baz], errors.keys
+    end
   end
 
   test "keys returns an empty array after try to get a message only" do
-    errors = ActiveModel::Errors.new(self)
+    errors = ActiveModel::Errors.new(Person.new)
     errors.messages[:foo]
     errors.messages[:baz]
 
-    assert_equal [], errors.keys
+    assert_deprecated do
+      assert_equal [], errors.keys
+    end
   end
 
   test "detecting whether there are errors with empty?, blank?, include?" do
@@ -146,32 +161,108 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal ["cannot be nil"], person.errors[:name]
   end
 
-  test "add an error message on a specific attribute" do
+  test "add an error message on a specific attribute (deprecated)" do
     person = Person.new
     person.errors.add(:name, "cannot be blank")
     assert_equal ["cannot be blank"], person.errors[:name]
   end
 
-  test "add an error message on a specific attribute with a defined type" do
+  test "add an error message on a specific attribute with a defined type (deprecated)" do
     person = Person.new
     person.errors.add(:name, :blank, message: "cannot be blank")
     assert_equal ["cannot be blank"], person.errors[:name]
   end
 
-  test "add an error with a symbol" do
+  test "add an error with a symbol (deprecated)" do
     person = Person.new
     person.errors.add(:name, :blank)
     message = person.errors.generate_message(:name, :blank)
     assert_equal [message], person.errors[:name]
   end
 
-  test "add an error with a proc" do
+  test "add an error with a proc (deprecated)" do
     person = Person.new
     message = Proc.new { "cannot be blank" }
     person.errors.add(:name, message)
     assert_equal ["cannot be blank"], person.errors[:name]
   end
 
+  test "add creates an error object and returns it" do
+    person = Person.new
+    error = person.errors.add(:name, :blank)
+
+    assert_equal :name, error.attribute
+    assert_equal :blank, error.type
+    assert_equal error, person.errors.objects.first
+  end
+
+  test "add, with type as symbol" do
+    person = Person.new
+    person.errors.add(:name, :blank)
+
+    assert_equal :blank, person.errors.objects.first.type
+    assert_equal ["can't be blank"], person.errors[:name]
+  end
+
+  test "add, with type as String" do
+    msg = "custom msg"
+
+    person = Person.new
+    person.errors.add(:name, msg)
+
+    assert_equal [msg], person.errors[:name]
+  end
+
+  test "add, with type as nil" do
+    person = Person.new
+    person.errors.add(:name)
+
+    assert_equal :invalid, person.errors.objects.first.type
+    assert_equal ["is invalid"], person.errors[:name]
+  end
+
+  test "add, with type as Proc, which evaluates to String" do
+    msg = "custom msg"
+    type = Proc.new { msg }
+
+    person = Person.new
+    person.errors.add(:name, type)
+
+    assert_equal [msg], person.errors[:name]
+  end
+
+  test "add, type being Proc, which evaluates to Symbol" do
+    type = Proc.new { :blank }
+
+    person = Person.new
+    person.errors.add(:name, type)
+
+    assert_equal :blank, person.errors.objects.first.type
+    assert_equal ["can't be blank"], person.errors[:name]
+  end
+
+  test "initialize options[:message] as Proc, which evaluates to String" do
+    msg = "custom msg"
+    type = Proc.new { msg }
+
+    person = Person.new
+    person.errors.add(:name, :blank, message: type)
+
+    assert_equal :blank, person.errors.objects.first.type
+    assert_equal [msg], person.errors[:name]
+  end
+
+  test "add, with options[:message] as Proc, which evaluates to String, where type is nil" do
+    msg = "custom msg"
+    type = Proc.new { msg }
+
+    person = Person.new
+    person.errors.add(:name, message: type)
+
+    assert_equal :invalid, person.errors.objects.first.type
+    assert_equal [msg], person.errors[:name]
+  end
+
   test "added? detects indifferent if a specific error was added to the object" do
     person = Person.new
     person.errors.add(:name, "cannot be blank")
@@ -437,6 +528,32 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal({ name: [{ error: :invalid }] }, person.errors.details)
   end
 
+  test "details retains original type as error" do
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, "cannot be nil")
+    errors.add("foo", "bar")
+    errors.add(:baz, nil)
+    errors.add(:age, :invalid, count: 3, message: "%{count} is too low")
+
+    assert_equal(
+      {
+        name: [{ error: "cannot be nil" }],
+        foo: [{ error: "bar" }],
+        baz: [{ error: nil }],
+        age: [{ error: :invalid, count: 3 }]
+      },
+      errors.details
+    )
+  end
+
+  test "group_by_attribute" do
+    person = Person.new
+    error = person.errors.add(:name, :invalid, message: "is bad")
+    hash = person.errors.group_by_attribute
+
+    assert_equal({ name: [error] }, hash)
+  end
+
   test "dup duplicates details" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)
@@ -449,7 +566,7 @@ class ErrorsTest < ActiveModel::TestCase
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)
     errors.delete(:name)
-    assert_empty errors.details[:name]
+    assert_not errors.added?(:name)
   end
 
   test "delete returns the deleted messages" do
@@ -467,7 +584,7 @@ class ErrorsTest < ActiveModel::TestCase
     assert_empty person.errors.details
   end
 
-  test "copy errors" do
+  test "copy errors (deprecated)" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)
     person = Person.new
@@ -477,7 +594,25 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal [:name], person.errors.details.keys
   end
 
-  test "merge errors" do
+  test "details returns empty array when accessed with non-existent attribute" do
+    errors = ActiveModel::Errors.new(Person.new)
+
+    assert_equal [], errors.details[:foo]
+  end
+
+  test "copy errors" do
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :invalid)
+    person = Person.new
+    person.errors.copy!(errors)
+
+    assert person.errors.added?(:name, :invalid)
+    person.errors.each do |error|
+      assert_same person, error.base
+    end
+  end
+
+  test "merge errors (deprecated)" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)
 
@@ -489,6 +624,18 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal({ name: [{ error: :blank }, { error: :invalid }] }, person.errors.details)
   end
 
+  test "merge errors" do
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :invalid)
+
+    person = Person.new
+    person.errors.add(:name, :blank)
+    person.errors.merge!(errors)
+
+    assert(person.errors.added?(:name, :invalid))
+    assert(person.errors.added?(:name, :blank))
+  end
+
   test "slice! removes all errors except the given keys" do
     person = Person.new
     person.errors.add(:name, "cannot be nil")
@@ -496,9 +643,9 @@ class ErrorsTest < ActiveModel::TestCase
     person.errors.add(:gender, "cannot be nil")
     person.errors.add(:city, "cannot be nil")
 
-    person.errors.slice!(:age, "gender")
+    assert_deprecated { person.errors.slice!(:age, "gender") }
 
-    assert_equal [:age, :gender], person.errors.keys
+    assert_equal [:age, :gender], assert_deprecated { person.errors.keys }
   end
 
   test "slice! returns the deleted errors" do
@@ -508,7 +655,7 @@ class ErrorsTest < ActiveModel::TestCase
     person.errors.add(:gender, "cannot be nil")
     person.errors.add(:city, "cannot be nil")
 
-    removed_errors = person.errors.slice!(:age, "gender")
+    removed_errors = assert_deprecated { person.errors.slice!(:age, "gender") }
 
     assert_equal({ name: ["cannot be nil"], city: ["cannot be nil"] }, removed_errors)
   end
@@ -518,10 +665,23 @@ class ErrorsTest < ActiveModel::TestCase
     errors.add(:name, :invalid)
     serialized = Marshal.load(Marshal.dump(errors))
 
+    assert_equal Person, serialized.instance_variable_get(:@base).class
     assert_equal errors.messages, serialized.messages
     assert_equal errors.details, serialized.details
   end
 
+  test "errors are compatible with marshal dumped from Rails 5.x" do
+    # Derived from
+    # errors = ActiveModel::Errors.new(Person.new)
+    # errors.add(:name, :invalid)
+    dump = "\x04\bU:\x18ActiveModel::Errors[\bo:\x17ErrorsTest::Person\x06:\f@errorsU;\x00[\b@\a{\x00{\x00{\x06:\tname[\x06I\"\x0Fis invalid\x06:\x06ET{\x06;\b[\x06{\x06:\nerror:\finvalid"
+    serialized = Marshal.load(dump)
+
+    assert_equal Person, serialized.instance_variable_get(:@base).class
+    assert_equal({ name: ["is invalid"] }, serialized.messages)
+    assert_equal({ name: [{ error: :invalid }] }, serialized.details)
+  end
+
   test "errors are backward compatible with the Rails 4.2 format" do
     yaml = <<~CODE
     --- !ruby/object:ActiveModel::Errors
@@ -541,4 +701,54 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal({}, errors.messages)
     assert_equal({}, errors.details)
   end
+
+  test "errors are compatible with YAML dumped from Rails 5.x" do
+    yaml = <<~CODE
+    --- !ruby/object:ActiveModel::Errors
+    base: &1 !ruby/object:ErrorsTest::Person
+      errors: !ruby/object:ActiveModel::Errors
+        base: *1
+        messages: {}
+        details: {}
+    messages:
+      :name:
+      - is invalid
+    details:
+      :name:
+      - :error: :invalid
+    CODE
+
+    errors = YAML.load(yaml)
+    assert_equal({ name: ["is invalid"] }, errors.messages)
+    assert_equal({ name: [{ error: :invalid }] }, errors.details)
+
+    errors.clear
+    assert_equal({}, errors.messages)
+    assert_equal({}, errors.details)
+  end
+
+  test "errors are compatible with YAML dumped from Rails 6.x" do
+    yaml = <<~CODE
+    --- !ruby/object:ActiveModel::Errors
+    base: &1 !ruby/object:ErrorsTest::Person
+      errors: !ruby/object:ActiveModel::Errors
+        base: *1
+        errors: []
+    errors:
+    - !ruby/object:ActiveModel::Error
+      base: *1
+      attribute: :name
+      type: :invalid
+      raw_type: :invalid
+      options: {}
+    CODE
+
+    errors = YAML.load(yaml)
+    assert_equal({ name: ["is invalid"] }, errors.messages)
+    assert_equal({ name: [{ error: :invalid }] }, errors.details)
+
+    errors.clear
+    assert_equal({}, errors.messages)
+    assert_equal({}, errors.details)
+  end
 end
diff --git a/activemodel/test/cases/nested_error_test.rb b/activemodel/test/cases/nested_error_test.rb
new file mode 100644
index 0000000000..5bad100da5
--- /dev/null
+++ b/activemodel/test/cases/nested_error_test.rb
@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "active_model/nested_error"
+require "models/topic"
+require "models/reply"
+
+class ErrorTest < ActiveModel::TestCase
+  def test_initialize
+    topic = Topic.new
+    inner_error = ActiveModel::Error.new(topic, :title, :not_enough, count: 2)
+    reply = Reply.new
+    error = ActiveModel::NestedError.new(reply, inner_error)
+
+    assert_equal reply, error.base
+    assert_equal inner_error.attribute, error.attribute
+    assert_equal inner_error.type, error.type
+    assert_equal(inner_error.options, error.options)
+  end
+
+  test "initialize with overriding attribute and type" do
+    topic = Topic.new
+    inner_error = ActiveModel::Error.new(topic, :title, :not_enough, count: 2)
+    reply = Reply.new
+    error = ActiveModel::NestedError.new(reply, inner_error, attribute: :parent, type: :foo)
+
+    assert_equal reply, error.base
+    assert_equal :parent, error.attribute
+    assert_equal :foo, error.type
+    assert_equal(inner_error.options, error.options)
+  end
+
+  def test_message
+    topic = Topic.new(author_name: "Bruce")
+    inner_error = ActiveModel::Error.new(topic, :title, :not_enough, message: Proc.new { |model, options|
+      "not good enough for #{model.author_name}"
+    })
+    reply = Reply.new(author_name: "Mark")
+    error = ActiveModel::NestedError.new(reply, inner_error)
+
+    assert_equal "not good enough for Bruce", error.message
+  end
+
+  def test_full_message
+    topic = Topic.new(author_name: "Bruce")
+    inner_error = ActiveModel::Error.new(topic, :title, :not_enough, message: Proc.new { |model, options|
+      "not good enough for #{model.author_name}"
+    })
+    reply = Reply.new(author_name: "Mark")
+    error = ActiveModel::NestedError.new(reply, inner_error)
+
+    assert_equal "Title not good enough for Bruce", error.full_message
+  end
+end
diff --git a/activemodel/test/cases/validations/i18n_validation_test.rb b/activemodel/test/cases/validations/i18n_validation_test.rb
index 35bb918f26..b7ee50832c 100644
--- a/activemodel/test/cases/validations/i18n_validation_test.rb
+++ b/activemodel/test/cases/validations/i18n_validation_test.rb
@@ -169,8 +169,8 @@ class I18nValidationTest < ActiveModel::TestCase
     # [ case,                              validation_options,            generate_message_options]
     [ "given no options",                  {},                            {}],
     [ "given custom message",              { message: "custom" },         { message: "custom" }],
-    [ "given if condition",                { if:                          lambda { true } },  {}],
-    [ "given unless condition",            { unless:                      lambda { false } }, {}],
+    [ "given if condition",                { if: lambda { true } },       {}],
+    [ "given unless condition",            { unless: lambda { false } },  {}],
     [ "given option that is not reserved", { format: "jpg" },             { format: "jpg" }]
   ]
 
@@ -181,6 +181,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title_confirmation, :confirmation, generate_message_options.merge(attribute: "Title")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -191,6 +192,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :accepted, generate_message_options]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -201,6 +203,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :blank, generate_message_options]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -211,6 +214,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :too_short, generate_message_options.merge(count: 3)]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -222,6 +226,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :too_long, generate_message_options.merge(count: 5)]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -232,6 +237,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :wrong_length, generate_message_options.merge(count: 5)]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -243,6 +249,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :invalid, generate_message_options.merge(value: "72x")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -254,6 +261,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :inclusion, generate_message_options.merge(value: "z")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -265,6 +273,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :inclusion, generate_message_options.merge(value: "z")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -276,6 +285,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :exclusion, generate_message_options.merge(value: "a")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -287,6 +297,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :exclusion, generate_message_options.merge(value: "a")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -298,6 +309,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :not_a_number, generate_message_options.merge(value: "a")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -309,6 +321,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :not_an_integer, generate_message_options.merge(value: "0.0")]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -320,6 +333,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :odd, generate_message_options.merge(value: 0)]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -331,6 +345,7 @@ class I18nValidationTest < ActiveModel::TestCase
       call = [:title, :less_than, generate_message_options.merge(value: 1, count: 0)]
       assert_called_with(@person.errors, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
diff --git a/activemodel/test/cases/validations/validations_context_test.rb b/activemodel/test/cases/validations/validations_context_test.rb
index 024eb1882f..3d2dea9828 100644
--- a/activemodel/test/cases/validations/validations_context_test.rb
+++ b/activemodel/test/cases/validations/validations_context_test.rb
@@ -14,13 +14,13 @@ class ValidationsContextTest < ActiveModel::TestCase
 
   class ValidatorThatAddsErrors < ActiveModel::Validator
     def validate(record)
-      record.errors[:base] << ERROR_MESSAGE
+      record.errors.add(:base, ERROR_MESSAGE)
     end
   end
 
   class AnotherValidatorThatAddsErrors < ActiveModel::Validator
     def validate(record)
-      record.errors[:base] << ANOTHER_ERROR_MESSAGE
+      record.errors.add(:base, ANOTHER_ERROR_MESSAGE)
     end
   end
 
diff --git a/activemodel/test/cases/validations/with_validation_test.rb b/activemodel/test/cases/validations/with_validation_test.rb
index 8239792c79..e6ae6603f2 100644
--- a/activemodel/test/cases/validations/with_validation_test.rb
+++ b/activemodel/test/cases/validations/with_validation_test.rb
@@ -14,13 +14,13 @@ class ValidatesWithTest < ActiveModel::TestCase
 
   class ValidatorThatAddsErrors < ActiveModel::Validator
     def validate(record)
-      record.errors[:base] << ERROR_MESSAGE
+      record.errors.add(:base, message: ERROR_MESSAGE)
     end
   end
 
   class OtherValidatorThatAddsErrors < ActiveModel::Validator
     def validate(record)
-      record.errors[:base] << OTHER_ERROR_MESSAGE
+      record.errors.add(:base, message: OTHER_ERROR_MESSAGE)
     end
   end
 
@@ -32,14 +32,14 @@ class ValidatesWithTest < ActiveModel::TestCase
   class ValidatorThatValidatesOptions < ActiveModel::Validator
     def validate(record)
       if options[:field] == :first_name
-        record.errors[:base] << ERROR_MESSAGE
+        record.errors.add(:base, message: ERROR_MESSAGE)
       end
     end
   end
 
   class ValidatorPerEachAttribute < ActiveModel::EachValidator
     def validate_each(record, attribute, value)
-      record.errors[attribute] << "Value is #{value}"
+      record.errors.add(attribute, message: "Value is #{value}")
     end
   end
 
diff --git a/activemodel/test/cases/validations_test.rb b/activemodel/test/cases/validations_test.rb
index 7776233db5..0b9e1b7005 100644
--- a/activemodel/test/cases/validations_test.rb
+++ b/activemodel/test/cases/validations_test.rb
@@ -53,7 +53,7 @@ class ValidationsTest < ActiveModel::TestCase
     r = Reply.new
     r.valid?
 
-    errors = r.errors.collect { |attr, messages| [attr.to_s, messages] }
+    errors = assert_deprecated { r.errors.collect { |attr, messages| [attr.to_s, messages] } }
 
     assert_includes errors, ["title", "is Empty"]
     assert_includes errors, ["content", "is Empty"]
@@ -74,7 +74,7 @@ class ValidationsTest < ActiveModel::TestCase
 
   def test_errors_on_nested_attributes_expands_name
     t = Topic.new
-    t.errors["replies.name"] << "can't be blank"
+    assert_deprecated { t.errors["replies.name"] << "can't be blank" }
     assert_equal ["Replies name can't be blank"], t.errors.full_messages
   end
 
@@ -216,7 +216,7 @@ class ValidationsTest < ActiveModel::TestCase
     t = Topic.new
     assert_predicate t, :invalid?
 
-    xml = t.errors.to_xml
+    xml = assert_deprecated { t.errors.to_xml }
     assert_match %r{<errors>}, xml
     assert_match %r{<error>Title can't be blank</error>}, xml
     assert_match %r{<error>Content can't be blank</error>}, xml
@@ -241,14 +241,14 @@ class ValidationsTest < ActiveModel::TestCase
     t = Topic.new title: ""
     assert_predicate t, :invalid?
 
-    assert_equal :title, key = t.errors.keys[0]
+    assert_equal :title, key = assert_deprecated { t.errors.keys[0] }
     assert_equal "can't be blank", t.errors[key][0]
     assert_equal "is too short (minimum is 2 characters)", t.errors[key][1]
-    assert_equal :author_name, key = t.errors.keys[1]
+    assert_equal :author_name, key = assert_deprecated { t.errors.keys[1] }
     assert_equal "can't be blank", t.errors[key][0]
-    assert_equal :author_email_address, key = t.errors.keys[2]
+    assert_equal :author_email_address, key = assert_deprecated { t.errors.keys[2] }
     assert_equal "will never be valid", t.errors[key][0]
-    assert_equal :content, key = t.errors.keys[3]
+    assert_equal :content, key = assert_deprecated { t.errors.keys[3] }
     assert_equal "is too short (minimum is 2 characters)", t.errors[key][0]
   end
 
diff --git a/activemodel/test/models/person_with_validator.rb b/activemodel/test/models/person_with_validator.rb
index 44e78cbc29..fbb28d2a0f 100644
--- a/activemodel/test/models/person_with_validator.rb
+++ b/activemodel/test/models/person_with_validator.rb
@@ -5,7 +5,7 @@ class PersonWithValidator
 
   class PresenceValidator < ActiveModel::EachValidator
     def validate_each(record, attribute, value)
-      record.errors[attribute] << "Local validator#{options[:custom]}" if value.blank?
+      record.errors.add(attribute, message: "Local validator#{options[:custom]}") if value.blank?
     end
   end
 
diff --git a/activemodel/test/models/reply.rb b/activemodel/test/models/reply.rb
index 6bb18f95fe..f340b6fb14 100644
--- a/activemodel/test/models/reply.rb
+++ b/activemodel/test/models/reply.rb
@@ -11,24 +11,24 @@ class Reply < Topic
   validate :check_wrong_update,     on: :update
 
   def check_empty_title
-    errors[:title] << "is Empty" unless title && title.size > 0
+    errors.add(:title, "is Empty") unless title && title.size > 0
   end
 
   def errors_on_empty_content
-    errors[:content] << "is Empty" unless content && content.size > 0
+    errors.add(:content, "is Empty") unless content && content.size > 0
   end
 
   def check_content_mismatch
     if title && content && content == "Mismatch"
-      errors[:title] << "is Content Mismatch"
+      errors.add(:title, "is Content Mismatch")
     end
   end
 
   def title_is_wrong_create
-    errors[:title] << "is Wrong Create" if title && title == "Wrong Create"
+    errors.add(:title, "is Wrong Create") if title && title == "Wrong Create"
   end
 
   def check_wrong_update
-    errors[:title] << "is Wrong Update" if title && title == "Wrong Update"
+    errors.add(:title, "is Wrong Update") if title && title == "Wrong Update"
   end
 end
diff --git a/activemodel/test/validators/email_validator.rb b/activemodel/test/validators/email_validator.rb
index 0c634d8659..774a10b2ba 100644
--- a/activemodel/test/validators/email_validator.rb
+++ b/activemodel/test/validators/email_validator.rb
@@ -2,7 +2,8 @@
 
 class EmailValidator < ActiveModel::EachValidator
   def validate_each(record, attribute, value)
-    record.errors[attribute] << (options[:message] || "is not an email") unless
-      /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i.match?(value)
+    unless /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i.match?(value)
+      record.errors.add(attribute, message: options[:message] || "is not an email")
+    end
   end
 end