Metaprogramming Q: Calling an external class method on after_save

[Debajit Adhikary]

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?

----------------------------------

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

[Debajit Adhikary]

(In the final code sample, the error I get is: 

NameError (undefined local variable or method `award_callback' for #<Post:0x002b57c04d52e0>): 

[Sonny Michaud]

You can probably do something like this:

class Post < ActiveRecord::Base
  after_save &AwardOne.method(:can_award?)
end

This will pass in the method as a block to ::after_save (#method returns a proc and the & will coerce it to a block).

- Sonny

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[Matt Jones]

On Thursday, 13 November 2014 23:37:40 UTC-5, Debajit Adhikary wrote:

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

You could do this with code like (not tested):

class AwardOne < AwardBase

  def self.trigger(klass, callback, method)

    us = self # this may not be needed

    klass.to_s.camelize.constantize.send(callback) do |record|

      us.send(method, record)

    end

  end

  trigger :post, :after_save, :can_award?

end

But seriously, DON'T DO THIS. Callbacks are already one of the more confusing features of AR, and confounding that by over-metaprogramming is going to obscure things further.

Even re-opening the Post class in award_one.rb and adding the callback in the standard style would be clearer than this, but I'd recommend just using the version you've already written (below). 

--Matt Jones