Scope of Constants in Ruby Modules

Question

I'm having a little problem with constant scope in mixin modules. Let's say I have something like this

module Auth    USER_KEY = "user" unless defined? USER_KEY    def authorize     user_id = session[USER_KEY]   def  end 

The USER_KEY constant should default to "user" unless it's already defined. Now I might mix this into a couple of places, but in one of those places the USER_KEY needs to be different, so we might have something like this

class ApplicationController < ActionController::Base    USER_KEY = "my_user"    include Auth    def test_auth     authorize   end  end 

I would expect that USER_KEY would be "my_user" when used in authorize, since it's already defined, but it's still "user", taken from the modules definition of USER_KEY. Anyone have any idea how to get authorize to use the classes version of USER_KEY?

Answer 1

The USER_KEY you declared (even conditionally) in Auth is globally known as Auth::USER_KEY. It doesn't get "mixed in" to including modules, though including modules can reference the key in a non-fully-qualified fashion.

If you want each including module (e.g. ApplicationController) to be able to define its own USER_KEY, try this:

module Auth   DEFAULT_USER_KEY = 'user'   def self.included(base)     unless base.const_defined?(:USER_KEY)       base.const_set :USER_KEY, Auth::DEFAULT_USER_KEY     end   end   def authorize     user_id = session[self.class.const_get(:USER_KEY)]   end end  class ApplicationController < ActionController::Base   USER_KEY = 'my_user'   include Auth end 

If you're going to go to all this trouble, though, you might as well just make it a class method:

module Auth   DEFAULT_USER_KEY = 'user'   def self.included(base)     base.extend Auth::ClassMethods     base.send :include, Auth::InstanceMethods   end   module ClassMethods     def user_key       Auth::DEFAULT_USER_KEY     end   end   module InstanceMethods     def authorize       user_id = session[self.class.user_key]     end   end end  class ApplicationController < ActionController::Base   def self.user_key     'my_user'   end end 

or a class-level accessor:

module Auth   DEFAULT_USER_KEY = 'user'   def self.included(base)     base.send :attr_accessor :user_key unless base.respond_to?(:user_key=)     base.user_key ||= Auth::DEFAULT_USER_KEY   end   def authorize     user_id = session[self.class.user_key]   end end  class ApplicationController < ActionController::Base   include Auth   self.user_key = 'my_user' end