Be able to pass a validator method to #validates

activemodel/lib/active_model/validations/with.rb

@@ -8,6 +8,12 @@ def _merge_attributes(attr_names)
         end
     end
 
+    class WithValidator < EachValidator
+      def validate_each(record, attr, val)
+        record.send options[:with]
+      end
+    end
+
     module ClassMethods
       # Passes the record off to the class or classes specified and allows them
       # to add errors based on more complex conditions.

activemodel/test/cases/validations/with_validation_test.rb

@@ -171,4 +171,16 @@ def check_validity!
     assert topic.errors[:title].empty?
     assert topic.errors[:content].empty?
   end
+
+  test "validates_with can validate with an instance method" do
+    Topic.validates :title, :with => :my_validation
+
+    topic = Topic.new :title => "foo"
+    assert topic.valid?
+    assert topic.errors[:title].empty?
+
+    topic = Topic.new
+    assert !topic.valid?
+    assert_equal ['is missing'], topic.errors[:title]
+  end
 end

activemodel/test/models/topic.rb

@@ -29,4 +29,8 @@ def perform_after_validation
     self.after_validation_performed = true
   end
 
+  def my_validation
+    errors.add :title, "is missing" unless title
+  end
+
 end