Add ActiveModel::Errors#merge!

ActiveModel::Errors#merge! allows ActiveModel::Errors to append errors from
a separate ActiveModel::Errors instance onto their own.

Example:

    person = Person.new
    person.errors.add(:name, :blank)

    errors = ActiveModel::Errors.new(Person.new)
    errors.add(:name, :invalid)

    person.errors.merge!(errors)
    puts person.errors.messages
    # => { name: ["can't be blank", "is invalid"] }

activemodel/CHANGELOG.md

+*   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.
 

activemodel/lib/active_model/errors.rb

@@ -93,6 +93,18 @@ def copy!(other) # :nodoc:
       @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"]

activemodel/test/cases/errors_test.rb

@@ -375,6 +375,18 @@ def test_no_key
     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)