Fix collisions with before and after validation callbacks.

This commit allows a user to do something like:
    before_validation :do_stuff, :on => [ :create, :update ]
    after_validation :do_more, :on => [ :create, :update ]

activemodel/lib/active_model/validations/callbacks.rb

@@ -56,7 +56,8 @@ module ActiveModel
           options = args.last
           if options.is_a?(Hash) && options[:on]
             options[:if] = Array(options[:if])
-            options[:if].unshift("self.validation_context == :#{options[:on]}")
+            options[:on] = Array(options[:on])
+            options[:if].unshift("self.validation_context.in? #{options[:on]}")
           end
           set_callback(:validation, :before, *args, &block)
         end
@@ -92,7 +93,10 @@ module ActiveModel
           options = args.extract_options!
           options[:prepend] = true
           options[:if] = Array(options[:if])
-          options[:if].unshift("self.validation_context == :#{options[:on]}") if options[:on]
+          if options[:on]
+            options[:on] = Array(options[:on])
+            options[:if].unshift("self.validation_context.in? #{options[:on]}")
+          end
           set_callback(:validation, :after, *(args << options), &block)
         end
       end

activerecord/test/cases/callbacks_test.rb

@@ -137,6 +137,32 @@ class OnCallbacksDeveloper < ActiveRecord::Base
   end
 end
 
+class ContextualCallbacksDeveloper < ActiveRecord::Base
+  self.table_name = 'developers'
+
+  before_validation { history << :before_validation }
+  before_validation :before_validation_on_create_and_update, :on => [ :create, :update ]
+
+  validate do
+    history << :validate
+  end
+
+  after_validation { history << :after_validation }
+  after_validation :after_validation_on_create_and_update, :on => [ :create, :update ]
+
+  def before_validation_on_create_and_update
+    history << "before_validation_on_#{self.validation_context}".to_sym
+  end
+
+  def after_validation_on_create_and_update
+    history << "after_validation_on_#{self.validation_context}".to_sym
+  end
+
+  def history
+    @history ||= []
+  end
+end
+
 class CallbackCancellationDeveloper < ActiveRecord::Base
   self.table_name = 'developers'
 
@@ -285,6 +311,17 @@ class CallbacksTest < ActiveRecord::TestCase
     ], david.history
   end
 
+  def test_validate_on_contextual_create
+    david = ContextualCallbacksDeveloper.create('name' => 'David', 'salary' => 1000000)
+    assert_equal [
+      :before_validation,
+      :before_validation_on_create,
+      :validate,
+      :after_validation,
+      :after_validation_on_create
+    ], david.history
+  end
+
   def test_update
     david = CallbackDeveloper.find(1)
     david.save
@@ -344,6 +381,18 @@ class CallbacksTest < ActiveRecord::TestCase
     ], david.history
   end
 
+  def test_validate_on_contextual_update
+    david = ContextualCallbacksDeveloper.find(1)
+    david.save
+    assert_equal [
+      :before_validation,
+      :before_validation_on_update,
+      :validate,
+      :after_validation,
+      :after_validation_on_update
+    ], david.history
+  end
+
   def test_destroy
     david = CallbackDeveloper.find(1)
     david.destroy