13 files changed, 57 insertions, 25 deletions
diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 2916e5eabb..048c43f2c4 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Add method `#merge!` for `ActiveModel::Errors`.
+
+    *Jahfer Husain*
+
 *   Fix regression in numericality validator when comparing Decimal and Float input
     values with more scale than the schema.
 
diff --git a/activemodel/lib/active_model.rb b/activemodel/lib/active_model.rb
index a2892e9ea9..ba1d2fbd44 100644
--- a/activemodel/lib/active_model.rb
+++ b/activemodel/lib/active_model.rb
@@ -23,7 +23,7 @@
 
 require "active_support"
 require "active_support/rails"
-require "active_model/version"
+require_relative "active_model/version"
 
 module ActiveModel
   extend ActiveSupport::Autoload
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 942b4fa9bb..76c23df541 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -93,6 +93,18 @@ module ActiveModel
       @details  = other.details.dup
     end
 
+    # Merges the errors from <tt>other</tt>.
+    #
+    # other - The ActiveModel::Errors instance.
+    #
+    # Examples
+    #
+    #   person.errors.merge!(other)
+    def merge!(other)
+      @messages.merge!(other.messages) { |_, ary1, ary2| ary1 + ary2 }
+      @details.merge!(other.details) { |_, ary1, ary2| ary1 + ary2 }
+    end
+
     # Clear the error messages.
     #
     #   person.errors.full_messages # => ["name cannot be nil"]
diff --git a/activemodel/lib/active_model/type.rb b/activemodel/lib/active_model/type.rb
index 095801d8f0..1741a67a4d 100644
--- a/activemodel/lib/active_model/type.rb
+++ b/activemodel/lib/active_model/type.rb
@@ -1,19 +1,19 @@
-require "active_model/type/helpers"
-require "active_model/type/value"
+require_relative "type/helpers"
+require_relative "type/value"
 
-require "active_model/type/big_integer"
-require "active_model/type/binary"
-require "active_model/type/boolean"
-require "active_model/type/date"
-require "active_model/type/date_time"
-require "active_model/type/decimal"
-require "active_model/type/float"
-require "active_model/type/immutable_string"
-require "active_model/type/integer"
-require "active_model/type/string"
-require "active_model/type/time"
+require_relative "type/big_integer"
+require_relative "type/binary"
+require_relative "type/boolean"
+require_relative "type/date"
+require_relative "type/date_time"
+require_relative "type/decimal"
+require_relative "type/float"
+require_relative "type/immutable_string"
+require_relative "type/integer"
+require_relative "type/string"
+require_relative "type/time"
 
-require "active_model/type/registry"
+require_relative "type/registry"
 
 module ActiveModel
   module Type
diff --git a/activemodel/lib/active_model/type/big_integer.rb b/activemodel/lib/active_model/type/big_integer.rb
index 3b629682fe..f461d7041e 100644
--- a/activemodel/lib/active_model/type/big_integer.rb
+++ b/activemodel/lib/active_model/type/big_integer.rb
@@ -1,4 +1,4 @@
-require "active_model/type/integer"
+require_relative "integer"
 
 module ActiveModel
   module Type
diff --git a/activemodel/lib/active_model/type/date_time.rb b/activemodel/lib/active_model/type/date_time.rb
index 5cb0077e45..8ad0daa9a6 100644
--- a/activemodel/lib/active_model/type/date_time.rb
+++ b/activemodel/lib/active_model/type/date_time.rb
@@ -10,6 +10,10 @@ module ActiveModel
         :datetime
       end
 
+      def serialize(value)
+        super(cast(value))
+      end
+
       private
 
         def cast_value(value)
diff --git a/activemodel/lib/active_model/type/helpers.rb b/activemodel/lib/active_model/type/helpers.rb
index 82cd9ebe98..1fe06ab3d5 100644
--- a/activemodel/lib/active_model/type/helpers.rb
+++ b/activemodel/lib/active_model/type/helpers.rb
@@ -1,4 +1,4 @@
-require "active_model/type/helpers/accepts_multiparameter_time"
-require "active_model/type/helpers/numeric"
-require "active_model/type/helpers/mutable"
-require "active_model/type/helpers/time_value"
+require_relative "helpers/accepts_multiparameter_time"
+require_relative "helpers/numeric"
+require_relative "helpers/mutable"
+require_relative "helpers/time_value"
diff --git a/activemodel/lib/active_model/type/string.rb b/activemodel/lib/active_model/type/string.rb
index 850cab962b..2fc027d3c4 100644
--- a/activemodel/lib/active_model/type/string.rb
+++ b/activemodel/lib/active_model/type/string.rb
@@ -1,4 +1,4 @@
-require "active_model/type/immutable_string"
+require_relative "immutable_string"
 
 module ActiveModel
   module Type
diff --git a/activemodel/lib/active_model/validations/exclusion.rb b/activemodel/lib/active_model/validations/exclusion.rb
index b7156ba802..d587079bd3 100644
--- a/activemodel/lib/active_model/validations/exclusion.rb
+++ b/activemodel/lib/active_model/validations/exclusion.rb
@@ -1,4 +1,4 @@
-require "active_model/validations/clusivity"
+require_relative "clusivity"
 
 module ActiveModel
   module Validations
diff --git a/activemodel/lib/active_model/validations/inclusion.rb b/activemodel/lib/active_model/validations/inclusion.rb
index c6c5bae649..74dac3b7df 100644
--- a/activemodel/lib/active_model/validations/inclusion.rb
+++ b/activemodel/lib/active_model/validations/inclusion.rb
@@ -1,4 +1,4 @@
-require "active_model/validations/clusivity"
+require_relative "clusivity"
 
 module ActiveModel
   module Validations
diff --git a/activemodel/test/cases/dirty_test.rb b/activemodel/test/cases/dirty_test.rb
index fdd18d7601..0242b96083 100644
--- a/activemodel/test/cases/dirty_test.rb
+++ b/activemodel/test/cases/dirty_test.rb
@@ -96,7 +96,7 @@ class DirtyTest < ActiveModel::TestCase
   end
 
   test "attribute mutation" do
-    @model.instance_variable_set("@name", "Yam")
+    @model.instance_variable_set("@name", "Yam".dup)
     assert !@model.name_changed?
     @model.name.replace("Hadad")
     assert !@model.name_changed?
diff --git a/activemodel/test/cases/errors_test.rb b/activemodel/test/cases/errors_test.rb
index 43aee5a814..7383b3e9b0 100644
--- a/activemodel/test/cases/errors_test.rb
+++ b/activemodel/test/cases/errors_test.rb
@@ -375,6 +375,18 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal [:name], person.errors.details.keys
   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_equal({ name: ["can't be blank", "is invalid"] }, person.errors.messages)
+    assert_equal({ name: [{ error: :blank }, { error: :invalid }] }, person.errors.details)
+  end
+
   test "errors are marshalable" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)
diff --git a/activemodel/test/cases/type/string_test.rb b/activemodel/test/cases/type/string_test.rb
index 47d412e27e..5bf9fc3527 100644
--- a/activemodel/test/cases/type/string_test.rb
+++ b/activemodel/test/cases/type/string_test.rb
@@ -14,7 +14,7 @@ module ActiveModel
       test "cast strings are mutable" do
         type = Type::String.new
 
-        s = "foo"
+        s = "foo".dup
         assert_equal false, type.cast(s).frozen?
         assert_equal false, s.frozen?