Question

First, let say that my understanding of the Ruby object model is woefully incomplete, having come from C, PHP, and Java.

Now, my application contains functionality that is shared by several models, so I’m trying to move that functionality into a module to keep things DRY. Each model has a ‘data’ attribute that needs to be serialized. However, when I move the serialize call into a module, I can’t make it work. Here is the basic model:

class MyModel < ActiveRecord::Base
  include MyApp::Data
end

And here is the the module:

module MyApp
  module Data
    serialize :data, ActiveRecord::Coders::Hstore

    def get_data(key)
      data && data[key]
    end

    def set_data(key, value)
      self.data = (data || {}).merge(key => value)
    end
  end
end

When I try to instantiate a model that includes MyApp:Data, I get the following error:

NoMethodError: undefined method `serialize' for MyApp::Data:Module

How do I make it so that when I instantiate MyModel, the data attribute is serialized by simply including MyApp::Data?

EDIT:

I’ve also tried this:

module MyApp
  module Data
    def self.included(base)
      base.extend(ClassMethods)
    end

    module ClassMethods
      serialize :data, ActiveRecord::Coders::Hstore
    end

    def get_data(key)
      data && data[key]
    end

    def set_data(key, value)
      self.data = (data || {}).merge(key => value)
    end
  end
end

Unfortunately, I’m still getting the same error.

Answer 1

As you have written it serialize is called when the module is loaded. The method call to serialize has no receiver so it is made on the Data module. You on the other hand want to call it on the class that is including the module.

You can do this with the self.included hook – it is called whenever the module is included and the class that is doing the including is passed as an argument, for example

module Data
  def self.included(base)
    base.serialize ...
  end
end

With activesupport you can also do

module Data
  extend ActiveSupport::Concern

  included do
    serialize :data, ...
  end
end

It sets up the self.included for you and evaluates the block such that self will be the including class, not the module (which can be handy if the methods you want to call happen to be protected/private – dsl stuff isn’t as readable when it’s full of base.send :blah)