Optionally pass in the attribute being validated to an instance method validator

activemodel/lib/active_model/validations/with.rb

@@ -10,7 +10,13 @@ module ActiveModel
 
     class WithValidator < EachValidator
       def validate_each(record, attr, val)
-        record.send options[:with]
+        method_name = options[:with]
+
+        if record.method(method_name).arity == 0
+          record.send method_name
+        else
+          record.send method_name, attr
+        end
       end
     end
 

activemodel/test/cases/validations/with_validation_test.rb

@@ -183,4 +183,13 @@ class ValidatesWithTest < ActiveModel::TestCase
     assert !topic.valid?
     assert_equal ['is missing'], topic.errors[:title]
   end
+
+  test "optionally pass in the attribute being validated when validating with an instance method" do
+    Topic.validates :title, :content, :with => :my_validation_with_arg
+
+    topic = Topic.new :title => "foo"
+    assert !topic.valid?
+    assert topic.errors[:title].empty?
+    assert_equal ['is missing'], topic.errors[:content]
+  end
 end

activemodel/test/models/topic.rb

@@ -33,4 +33,8 @@ class Topic
     errors.add :title, "is missing" unless title
   end
 
+  def my_validation_with_arg(attr)
+    errors.add attr, "is missing" unless send(attr)
+  end
+
 end