Question

I always have a brain cramp when it comes to this. I’m creating a module to mix in to model-like classes but it needs to keep exactly one copy of serializable attributes per class. So here is the code (that doesn’t work).

module Checkin
  module Model
    def self.included(base)
      base.extend(ClassMethods)
    end

    @@serialiable_attrs = []   <== really not the right place
    module ClassMethods
      def serializable(*attrs)
        attrs.each{|attr| @@serializable_attrs << attr} # Is this @@ or just @?
      end
    end

    def serialize!
      @@serializable_attrs.each{|a| do_something_with(a)} # <== Will this work?
    end
  end
end

class Person  
  include Checkin::Model
  serializable :first_name, :original_name, :last_name, :checked_in, :people_attending
  # etc., etc.
end

What I’m wrangling with are two things:

1. How to define my mix-in such that a class variable magically springs into existence; and
2. How to access that variable both in my ClassMethods module and in the (for lack of a better term) instance methods part of the module.

Note that I’ve settled on a mix-in technique rather than inheritance because I will have Validatable, Persistable, Serializable and so on. Also, I know there are all sorts of validation and persistence layers available that are well tested and heavily used. This is a different beast and I really should know who to do this in my sleep, right?

Any Ruby wizards who can help me understand how to do this or suggest a different direction to approach this problem from, I appreciate the help!

Answer 1

Try removing the class variable, and adding this to the module ClassMethod:

def self.extended(klass)
  klass.instance_variable_set("@serializable_attrs", [])
end

And changing the double-@ to single in serializable. And change serialize! to this:

self.class.instance_variable_get("@serializable_attrs").each{|a| do_something_with(a)}