http://icelab.com.au/articles/embrace-the-metaclass-and-extend-your-activemodels/

activesupport_callbacks_fix.rb

module ActiveSupport
  module Callbacks
    module ClassMethods
      def define_callbacks(*callbacks)
        config = callbacks.last.is_a?(Hash) ? callbacks.pop : {}
        callbacks.each do |callback|
          class_attribute "_#{callback}_callbacks"
          send("_#{callback}_callbacks=", CallbackChain.new(callback, config))
          __define_runner(callback)
        end
      end
    end
  end
end