From: Rick DeNatale <rick.denatale@...>
Date: 2009-12-08T08:35:48+09:00
Subject: Re: Use of extend <module> from within a Class

On Mon, Dec 7, 2009 at 4:26 PM, Mike Papper <bodarotech@gmail.com> wrote:
> Thanks for the clarification and sample.
>
> So, I cannot simply extend the ActionController::UrlWriter module in my
> class because that module does this:
>
> self.class.default_url_options.merge(options)
>
> However, within a rails controller, this module can be included as a
> "helper". Does anyone know how the magic rails helper function makes
> this call to self.class.default_url work? I've looked at the code for
> helper but cannot really make any sense of it.

MyController < ActionController::Base
   include UrlWriter  # include not extend
end

If you read the code in actionpack/lib/action_controller/helpers.rb it
really isn't that hard to see what's going on

if in the controller you write say

   helper :url_writer

then the helper method takes the case branch for a symbol. which is:

              file_name  = arg.to_s.underscore + '_helper'
              class_name = file_name.camelize

              begin
                require_dependency(file_name)
              rescue LoadError => load_error
                requiree = / -- (.*?)(\.rb)?$/.match(load_error.message).to_a[1]
                if requiree == file_name
                  msg = "Missing helper file helpers/#{file_name}.rb"
                  raise LoadError.new(msg).copy_blame!(load_error)
                else
                  raise
                end
              end

              add_template_helper(class_name.constantize)

So it computes a file name from the argument, in this case
"url_writer.rb", and a class (actually a Module) name "UrlWriter"

Then require dependency loads the file if necessary, and finally.

class_name.constantize turns teh class name into the actual module,
and passes it as the argument to add_template_helper which is defined
above as:

      def add_template_helper(helper_module) #:nodoc:
        master_helper_module.module_eval { include helper_module }
      end

So it is effectively the same thing as what I first wrote.

-- 
Rick DeNatale

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