Metaprogramming Q: Calling an external class method on after_save

rubyonrails-talk
1

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?

2

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

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

3

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

4

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