From: "Christopher J. Bottaro" <cjbottaro@...>
Date: 2012-02-10T06:49:53+09:00
Subject: Re: Introducing Schizo, A DCI gem

Andrew W. wrote in post #1045018:
> Fantastic to see some work being done on this, kudos to you! I have a
> dumb
> question. In the example in the readme, why can't you just do
> object.singleton_class.class_eval instead of object.class.class_eval?
> That
> should make the changes only affect that object, right?

Hehe, I think I should add this to the README...

Originally, that's how I tried to use DCI with ActiveRecord.  I tried 
something like this:

module Poster
  def self.extended(object)
    object.singleton_class.class_eval do
      after_save do
        update_posts_or_something
      end
    end
  end

  def make_post
    # blah, do something
  end
end

user = User.find(1)
user.extend(Poster)
user.name = "Callie"
user.save

Well, it doesn't work.  ActiveSupport's callbacks work by looking at the 
object's class for any defined callbacks, it never looks at the 
singleton class.

The good news is that a single, one line change to ActiveSupport fixes 
the problem.  The bad news is that any object that has a singleton class 
defined can't be marshaled.  The Rails core team doesn't like that (a 
long while ago I submitted a patch that passed all tests, but created 
singleton classes on ActiveRecord instances... the accepted it 
initially, but then backed it out once they found out about the 
Marshaling issue).

I think one of the biggest downsides to not being able to marshal an 
object is that you can't cache it.

Ok, so regardless of the marshaling issue, let's say getting callbacks 
to work is as simple as that one line change to ActiveSupport.  Great 
let's move on to associations...

Long story short, it's far from a one line change.  Associations are 
complicated, messy and magical.

I gave up trying to get object.singleton_class.class_eval(...) to work 
with AS/AR and chose the approach you see in Schizo.

-- C

-- 
Posted via http://www.ruby-forum.com/.