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

Constants don't have global scope in Ruby. Constants can be visible from any scope, but you must specify where the constant is to be found. When you begin a new class, module, or def, you begin a new scope, and if you want a constant from another scope, you have to specify where to find it.

class C
  X = 1
  module M
    X = 2
    class D
      X = 3
      puts X          # => 3
      puts C::X       # => 1
      puts C::M::X    # => 2
      puts M::X       # => 2    
    end
  end
end

Answer 2

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