Ruby on Rails

I have the following classes:

    class AwardBase

    class AwardOne < AwardBase

    class Post < ActiveRecord::Base

The Post is an ActiveRecord, and the Award has a can_award? class method which takes a post object and checks to see if it meets some criteria. If yes, it updates post.owner.awards.

I know I can do this using an Observer pattern (I tested it and the code works fine). However, that requires me to add additional code to the model. I'd like not to touch the model at all if possible. What I'd like to do is run the Award checks like this (the trigger will be invoked at class load time):

    class AwardOne < AwardBase

      trigger :post, :after_save

      def self.can_award?(post)

        ...

      end

    end

The intention with the above code is that it should automatically add AwardOne.can_award? to Post's after_save method

So essentially what I'm trying to do is to get the above code to be equivalent to:

    class Post < ActiveRecord::Base

      after_save AwardOne.can_award?(self)

      ...

    end

which is basically:

    class Post < ActiveRecord::Base

      after_save :check_award

      def check_award

        AwardOne.can_award?(self)

      end

    end

How can I do this without modifying the Post class?

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Here's what I've done (which does not appear to work):

    class AwardBase

      def self.trigger (klass, active_record_event)

        model_class = klass.to_class

        this = self

        model_class.instance_eval do

          def award_callback

            this.can_award?(self)

          end

        end

        model_class.class_eval do

          self.send(active_record_event, :award_callback)

        end

      end

      def self.can_award? (model)

        raise NotImplementedError

      end

    end