16 files changed, 696 insertions, 114 deletions
diff --git a/activemodel/test/cases/attribute_methods_test.rb b/activemodel/test/cases/attribute_methods_test.rb
index 4e228032c3..e3b3b15f25 100644
--- a/activemodel/test/cases/attribute_methods_test.rb
+++ b/activemodel/test/cases/attribute_methods_test.rb
@@ -40,7 +40,6 @@ private
   end
 
 protected
-
   def protected_method
     "O_o O_o"
   end
diff --git a/activemodel/test/cases/attributes_test.rb b/activemodel/test/cases/attributes_test.rb
index 5483fb101d..af0ddcb92f 100644
--- a/activemodel/test/cases/attributes_test.rb
+++ b/activemodel/test/cases/attributes_test.rb
@@ -67,6 +67,20 @@ module ActiveModel
       assert_equal expected_attributes, data.attributes
     end
 
+    test "reading attribute names" do
+      names = [
+        "integer_field",
+        "string_field",
+        "decimal_field",
+        "string_with_default",
+        "date_field",
+        "boolean_field"
+      ]
+
+      assert_equal names, ModelForAttributesTest.attribute_names
+      assert_equal names, ModelForAttributesTest.new.attribute_names
+    end
+
     test "nonexistent attribute" do
       assert_raise ActiveModel::UnknownAttributeError do
         ModelForAttributesTest.new(nonexistent: "nonexistent")
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..a6cd1da717 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 +31,66 @@ 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_each_when_arity_is_negative
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :blank)
+    errors.add(:gender, :blank)
+
+    assert_equal([:name, :gender], errors.map(&:attribute))
+  end
+
+  def test_any?
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name)
+    assert_not_deprecated {
+      assert errors.any?, "any? should return true"
+    }
+    assert_not_deprecated {
+      assert errors.any? { |_| true }, "any? should return true"
+    }
+  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 +104,58 @@ 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 +180,130 @@ 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? when attribute was added through a collection" do
+    person = Person.new
+    person.errors.add(:"family_members.name", :too_long, count: 25)
+    assert person.errors.added?(:"family_members.name", :too_long, count: 25)
+    assert_not person.errors.added?(:"family_members.name", :too_long)
+    assert_not person.errors.added?(:"family_members.name", :too_long, name: "hello")
+  end
+
+  test "added? ignores callback option" do
+    person = Person.new
+
+    person.errors.add(:name, :too_long, if: -> { true })
+    assert person.errors.added?(:name, :too_long)
+  end
+
+  test "added? ignores message option" do
+    person = Person.new
+
+    person.errors.add(:name, :too_long, message: proc { "foo" })
+    assert person.errors.added?(:name, :too_long)
+  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")
@@ -342,6 +474,17 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal ["name cannot be blank", "name cannot be nil"], person.errors.to_a
   end
 
+  test "to_h is deprecated" do
+    person = Person.new
+    person.errors.add(:name, "cannot be blank")
+    person.errors.add(:name, "too long")
+
+    expected_deprecation = "ActiveModel::Errors#to_h is deprecated"
+    assert_deprecated(expected_deprecation) do
+      assert_equal({ name: "too long" }, person.errors.to_h)
+    end
+  end
+
   test "to_hash returns the error messages hash" do
     person = Person.new
     person.errors.add(:name, "cannot be blank")
@@ -358,6 +501,27 @@ class ErrorsTest < ActiveModel::TestCase
     assert_nil person.errors.as_json.default_proc
   end
 
+  test "full_messages doesn't require the base object to respond to `:errors" do
+    model = Class.new do
+      def initialize
+        @errors = ActiveModel::Errors.new(self)
+        @errors.add(:name, "bar")
+      end
+
+      def self.human_attribute_name(attr, options = {})
+        "foo"
+      end
+
+      def call
+        error_wrapper = Struct.new(:model_errors)
+
+        error_wrapper.new(@errors)
+      end
+    end
+
+    assert_equal(["foo bar"], model.new.call.model_errors.full_messages)
+  end
+
   test "full_messages creates a list of error messages with the attribute name included" do
     person = Person.new
     person.errors.add(:name, "cannot be blank")
@@ -437,6 +601,32 @@ 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)
@@ -445,11 +635,17 @@ class ErrorsTest < ActiveModel::TestCase
     assert_not_equal errors_dup.details, errors.details
   end
 
+  test "delete returns nil when no errors were deleted" do
+    errors = ActiveModel::Errors.new(Person.new)
+
+    assert_nil(errors.delete(:name))
+  end
+
   test "delete removes details on given attribute" do
     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 +663,7 @@ 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 +673,25 @@ 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 +703,18 @@ 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 +722,9 @@ 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 +734,7 @@ 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 +744,23 @@ 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 +780,54 @@ 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..6c2458ba83
--- /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 NestedErrorTest < 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/railtie_test.rb b/activemodel/test/cases/railtie_test.rb
index 95ee7cace3..f5ff1a3fd7 100644
--- a/activemodel/test/cases/railtie_test.rb
+++ b/activemodel/test/cases/railtie_test.rb
@@ -35,20 +35,20 @@ class RailtieTest < ActiveModel::TestCase
   test "i18n customize full message defaults to false" do
     @app.initialize!
 
-    assert_equal false, ActiveModel::Errors.i18n_customize_full_message
+    assert_equal false, ActiveModel::Error.i18n_customize_full_message
   end
 
   test "i18n customize full message can be disabled" do
     @app.config.active_model.i18n_customize_full_message = false
     @app.initialize!
 
-    assert_equal false, ActiveModel::Errors.i18n_customize_full_message
+    assert_equal false, ActiveModel::Error.i18n_customize_full_message
   end
 
   test "i18n customize full message can be enabled" do
     @app.config.active_model.i18n_customize_full_message = true
     @app.initialize!
 
-    assert_equal true, ActiveModel::Errors.i18n_customize_full_message
+    assert_equal true, ActiveModel::Error.i18n_customize_full_message
   end
 end
diff --git a/activemodel/test/cases/type/date_time_test.rb b/activemodel/test/cases/type/date_time_test.rb
index 74b47d1b4d..4a63eee0cf 100644
--- a/activemodel/test/cases/type/date_time_test.rb
+++ b/activemodel/test/cases/type/date_time_test.rb
@@ -37,7 +37,6 @@ module ActiveModel
       end
 
       private
-
         def with_timezone_config(default:)
           old_zone_default = ::Time.zone_default
           ::Time.zone_default = ::Time.find_zone(default)
diff --git a/activemodel/test/cases/validations/acceptance_validation_test.rb b/activemodel/test/cases/validations/acceptance_validation_test.rb
index 72baf6e7a7..6bd3d292f8 100644
--- a/activemodel/test/cases/validations/acceptance_validation_test.rb
+++ b/activemodel/test/cases/validations/acceptance_validation_test.rb
@@ -92,7 +92,6 @@ class AcceptanceValidationTest < ActiveModel::TestCase
   end
 
   private
-
     # Acceptance validator includes anonymous module into class, which cannot
     # be cleared, so to avoid multiple inclusions we use a named subclass which
     # we can remove in teardown.
diff --git a/activemodel/test/cases/validations/i18n_validation_test.rb b/activemodel/test/cases/validations/i18n_validation_test.rb
index 35bb918f26..c81649f493 100644
--- a/activemodel/test/cases/validations/i18n_validation_test.rb
+++ b/activemodel/test/cases/validations/i18n_validation_test.rb
@@ -13,8 +13,8 @@ class I18nValidationTest < ActiveModel::TestCase
     I18n.backend = I18n::Backend::Simple.new
     I18n.backend.store_translations("en", errors: { messages: { custom: nil } })
 
-    @original_i18n_customize_full_message = ActiveModel::Errors.i18n_customize_full_message
-    ActiveModel::Errors.i18n_customize_full_message = true
+    @original_i18n_customize_full_message = ActiveModel::Error.i18n_customize_full_message
+    ActiveModel::Error.i18n_customize_full_message = true
   end
 
   def teardown
@@ -24,7 +24,7 @@ class I18nValidationTest < ActiveModel::TestCase
     I18n.load_path.replace @old_load_path
     I18n.backend = @old_backend
     I18n.backend.reload!
-    ActiveModel::Errors.i18n_customize_full_message = @original_i18n_customize_full_message
+    ActiveModel::Error.i18n_customize_full_message = @original_i18n_customize_full_message
   end
 
   def test_full_message_encoding
@@ -49,7 +49,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_doesnt_use_attribute_format_without_config
-    ActiveModel::Errors.i18n_customize_full_message = false
+    ActiveModel::Error.i18n_customize_full_message = false
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { person: { attributes: { name: { format: "%{message}" } } } } } })
@@ -59,8 +59,21 @@ class I18nValidationTest < ActiveModel::TestCase
     assert_equal "Name test cannot be blank", person.errors.full_message(:name_test, "cannot be blank")
   end
 
+  def test_errors_full_messages_on_nested_error_uses_attribute_format
+    ActiveModel::Error.i18n_customize_full_message = true
+    I18n.backend.store_translations("en", activemodel: {
+      errors: { models: { person: { attributes: { gender: "Gender" } } } },
+      attributes: { "person/contacts": { gender: "Gender" } }
+    })
+
+    person = person_class.new
+    error = ActiveModel::Error.new(person, :gender, "can't be blank")
+    person.errors.import(error, attribute: "person[0].contacts.gender")
+    assert_equal ["Gender can't be blank"], person.errors.full_messages
+  end
+
   def test_errors_full_messages_uses_attribute_format
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { person: { attributes: { name: { format: "%{message}" } } } } } })
@@ -71,7 +84,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_uses_model_format
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { person: { format: "%{message}" } } } })
@@ -82,7 +95,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_uses_deeply_nested_model_attributes_format
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { 'person/contacts/addresses': { attributes: { street: { format: "%{message}" } } } } } })
@@ -93,7 +106,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_uses_deeply_nested_model_model_format
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { 'person/contacts/addresses': { format: "%{message}" } } } })
@@ -104,7 +117,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_with_indexed_deeply_nested_attributes_and_attributes_format
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { 'person/contacts/addresses': { attributes: { street: { format: "%{message}" } } } } } })
@@ -115,7 +128,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_with_indexed_deeply_nested_attributes_and_model_format
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { 'person/contacts/addresses': { format: "%{message}" } } } })
@@ -126,7 +139,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_with_indexed_deeply_nested_attributes_and_i18n_attribute_name
-    ActiveModel::Errors.i18n_customize_full_message = true
+    ActiveModel::Error.i18n_customize_full_message = true
 
     I18n.backend.store_translations("en", activemodel: {
       attributes: { 'person/contacts/addresses': { country: "Country" } }
@@ -138,7 +151,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_with_indexed_deeply_nested_attributes_without_i18n_config
-    ActiveModel::Errors.i18n_customize_full_message = false
+    ActiveModel::Error.i18n_customize_full_message = false
 
     I18n.backend.store_translations("en", activemodel: {
       errors: { models: { 'person/contacts/addresses': { attributes: { street: { format: "%{message}" } } } } } })
@@ -149,7 +162,7 @@ class I18nValidationTest < ActiveModel::TestCase
   end
 
   def test_errors_full_messages_with_i18n_attribute_name_without_i18n_config
-    ActiveModel::Errors.i18n_customize_full_message = false
+    ActiveModel::Error.i18n_customize_full_message = false
 
     I18n.backend.store_translations("en", activemodel: {
       attributes: { 'person/contacts[0]/addresses[0]': { country: "Country" } }
@@ -169,8 +182,8 @@ 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" }]
   ]
 
@@ -178,9 +191,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_confirmation_of on generated message #{name}" do
       person_class.validates_confirmation_of :title, validation_options
       @person.title_confirmation = "foo"
-      call = [:title_confirmation, :confirmation, generate_message_options.merge(attribute: "Title")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title_confirmation, :confirmation, @person, generate_message_options.merge(attribute: "Title")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -188,9 +202,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_acceptance_of on generated message #{name}" do
       person_class.validates_acceptance_of :title, validation_options.merge(allow_nil: false)
-      call = [:title, :accepted, generate_message_options]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :accepted, @person, generate_message_options]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -198,9 +213,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_presence_of on generated message #{name}" do
       person_class.validates_presence_of :title, validation_options
-      call = [:title, :blank, generate_message_options]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :blank, @person, generate_message_options]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -208,9 +224,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_length_of for :within on generated message when too short #{name}" do
       person_class.validates_length_of :title, validation_options.merge(within: 3..5)
-      call = [:title, :too_short, generate_message_options.merge(count: 3)]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :too_short, @person, generate_message_options.merge(count: 3)]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -219,9 +236,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_length_of for :too_long generated message #{name}" do
       person_class.validates_length_of :title, validation_options.merge(within: 3..5)
       @person.title = "this title is too long"
-      call = [:title, :too_long, generate_message_options.merge(count: 5)]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :too_long, @person, generate_message_options.merge(count: 5)]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -229,9 +247,10 @@ class I18nValidationTest < ActiveModel::TestCase
   COMMON_CASES.each do |name, validation_options, generate_message_options|
     test "validates_length_of for :is on generated message #{name}" do
       person_class.validates_length_of :title, validation_options.merge(is: 5)
-      call = [:title, :wrong_length, generate_message_options.merge(count: 5)]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :wrong_length, @person, generate_message_options.merge(count: 5)]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -240,9 +259,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_format_of on generated message #{name}" do
       person_class.validates_format_of :title, validation_options.merge(with: /\A[1-9][0-9]*\z/)
       @person.title = "72x"
-      call = [:title, :invalid, generate_message_options.merge(value: "72x")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :invalid, @person, generate_message_options.merge(value: "72x")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -251,9 +271,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_inclusion_of on generated message #{name}" do
       person_class.validates_inclusion_of :title, validation_options.merge(in: %w(a b c))
       @person.title = "z"
-      call = [:title, :inclusion, generate_message_options.merge(value: "z")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :inclusion, @person, generate_message_options.merge(value: "z")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -262,9 +283,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_inclusion_of using :within on generated message #{name}" do
       person_class.validates_inclusion_of :title, validation_options.merge(within: %w(a b c))
       @person.title = "z"
-      call = [:title, :inclusion, generate_message_options.merge(value: "z")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :inclusion, @person, generate_message_options.merge(value: "z")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -273,9 +295,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_exclusion_of generated message #{name}" do
       person_class.validates_exclusion_of :title, validation_options.merge(in: %w(a b c))
       @person.title = "a"
-      call = [:title, :exclusion, generate_message_options.merge(value: "a")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :exclusion, @person, generate_message_options.merge(value: "a")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -284,9 +307,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_exclusion_of using :within generated message #{name}" do
       person_class.validates_exclusion_of :title, validation_options.merge(within: %w(a b c))
       @person.title = "a"
-      call = [:title, :exclusion, generate_message_options.merge(value: "a")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :exclusion, @person, generate_message_options.merge(value: "a")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -295,9 +319,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of generated message #{name}" do
       person_class.validates_numericality_of :title, validation_options
       @person.title = "a"
-      call = [:title, :not_a_number, generate_message_options.merge(value: "a")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :not_a_number, @person, generate_message_options.merge(value: "a")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -306,9 +331,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of for :only_integer on generated message #{name}" do
       person_class.validates_numericality_of :title, validation_options.merge(only_integer: true)
       @person.title = "0.0"
-      call = [:title, :not_an_integer, generate_message_options.merge(value: "0.0")]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :not_an_integer, @person, generate_message_options.merge(value: "0.0")]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -317,9 +343,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of for :odd on generated message #{name}" do
       person_class.validates_numericality_of :title, validation_options.merge(only_integer: true, odd: true)
       @person.title = 0
-      call = [:title, :odd, generate_message_options.merge(value: 0)]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :odd, @person, generate_message_options.merge(value: 0)]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
@@ -328,9 +355,10 @@ class I18nValidationTest < ActiveModel::TestCase
     test "validates_numericality_of for :less_than on generated message #{name}" do
       person_class.validates_numericality_of :title, validation_options.merge(only_integer: true, less_than: 0)
       @person.title = 1
-      call = [:title, :less_than, generate_message_options.merge(value: 1, count: 0)]
-      assert_called_with(@person.errors, :generate_message, call) do
+      call = [:title, :less_than, @person, generate_message_options.merge(value: 1, count: 0)]
+      assert_called_with(ActiveModel::Error, :generate_message, call) do
         @person.valid?
+        @person.errors.messages
       end
     end
   end
diff --git a/activemodel/test/cases/validations/numericality_validation_test.rb b/activemodel/test/cases/validations/numericality_validation_test.rb
index 16c44762cb..191af033df 100644
--- a/activemodel/test/cases/validations/numericality_validation_test.rb
+++ b/activemodel/test/cases/validations/numericality_validation_test.rb
@@ -310,7 +310,6 @@ class NumericalityValidationTest < ActiveModel::TestCase
   end
 
   private
-
     def invalid!(values, error = nil)
       with_each_topic_approved_value(values) do |topic, value|
         assert topic.invalid?, "#{value.inspect} not rejected as a number"
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