Add convenience method group_by_attribute

Many operations need grouping of errors by attributes, e.g. ActiveRecord::AutosaveAssociation#association_valid?

Refactor other methods using group_by_attribute

activemodel/lib/active_model/errors.rb

@@ -292,18 +292,9 @@ module ActiveModel
     #   person.errors.to_hash(true) # => {:name=>["name cannot be nil"]}
     def to_hash(full_messages = false)
       hash = {}
-      @errors.each do |error|
-        if full_messages
-          message = error.full_message
-        else
-          message = error.message
-        end
-
-        if hash.has_key?(error.attribute)
-          hash[error.attribute] << message
-        else
-          hash[error.attribute] = [message]
-        end
+      message_method = full_messages ? :full_message : :message
+      group_by_attribute.each do |attribute, errors|
+        hash[attribute] = errors.map(&message_method)
       end
       hash
     end
@@ -311,18 +302,16 @@ module ActiveModel
 
     def details
       hash = {}
-      @errors.each do |error|
-        detail = error.detail
-
-        if hash.has_key?(error.attribute)
-          hash[error.attribute] << detail
-        else
-          hash[error.attribute] = [detail]
-        end
+      group_by_attribute.each do |attribute, errors|
+        hash[attribute] = errors.map(&:detail)
       end
       hash
     end
 
+    def group_by_attribute
+      group_by(&:attribute)
+    end
+
     # Adds +message+ to the error messages and used validator type to +details+ on +attribute+.
     # More than one error can be added to the same +attribute+.
     # If no +message+ is supplied, <tt>:invalid</tt> is assumed.

activemodel/test/cases/errors_test.rb

@@ -494,6 +494,14 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal({ name: [{ error: :invalid }] }, person.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)