Provide a way to specify alternate option keys for validates

activemodel/lib/active_model/validations/validates.rb

@@ -81,7 +81,7 @@ module ClassMethods
       #
       def validates(*attributes)
         defaults = attributes.extract_options!
-        validations = defaults.slice!(:if, :unless, :on, :allow_blank, :allow_nil)
+        validations = defaults.slice!(*_validates_default_keys)
 
         raise ArgumentError, "You need to supply at least one attribute" if attributes.empty?
         raise ArgumentError, "You need to supply at least one validation" if validations.empty?
@@ -103,6 +103,12 @@ def validates(*attributes)
 
     protected
 
+      # When creating custom validators, it might be useful to be able to specify
+      # additional default keys. This can be done by overwriting this method.
+      def _validates_default_keys
+        [ :if, :unless, :on, :allow_blank, :allow_nil ]
+      end
+
       def _parse_validates_options(options) #:nodoc:
         case options
         when TrueClass

activemodel/test/cases/validations/validates_test.rb

 # encoding: utf-8
 require 'cases/helper'
 require 'models/person'
+require 'models/topic'
 require 'models/person_with_validator'
 require 'validators/email_validator'
 require 'validators/namespace/email_validator'
@@ -11,6 +12,7 @@ class ValidatesTest < ActiveModel::TestCase
 
   def reset_callbacks
     Person.reset_callbacks(:validate)
+    Topic.reset_callbacks(:validate)
     PersonWithValidator.reset_callbacks(:validate)
   end
 
@@ -139,4 +141,13 @@ def test_validates_with_included_validator_and_wildcard_shortcut
     person.valid?
     assert_equal ['does not appear to be like Mr.'], person.errors[:title]
   end
+
+  def test_defining_extra_default_keys_for_validates
+    Topic.validates :title, :confirmation => true, :message => 'Y U NO CONFIRM'
+    topic = Topic.new
+    topic.title = "What's happening"
+    topic.title_confirmation = "Not this"
+    assert !topic.valid?
+    assert_equal ['Y U NO CONFIRM'], topic.errors[:title]
+  end
 end

activemodel/test/models/topic.rb

@@ -2,6 +2,10 @@ class Topic
   include ActiveModel::Validations
   include ActiveModel::Validations::Callbacks
 
+  def self._validates_default_keys
+    super | [ :message ]
+  end
+
   attr_accessor :title, :author_name, :content, :approved
   attr_accessor :after_validation_performed