title: Rails 4 concerns: Give class instance variables to model
body: <p>With Rails concerns I can give my model class methods and instance methods through modules by including them. No blog entry or thread that I've found mentions how I can include variables in my model though.</p>
  
  <p>Specifically I would like to give my including model a class instance variable <code>@question</code>, but I don't know where to put the declaration in the module so it is applied. I would also like the class instance variable to be overridden if  the model itself declares that variable.</p>
  
  <p>Does the <code>ActiveSupport::Concern</code> module actually care about variables at all?</p>
  
  <pre><code>module ContentAttribute
      extend ActiveSupport::Concern
  
      def foo
          p "hi"
      end
  
      module ClassMethods
  
          # @question = "I am a generic question." [doesn't work]
  
          def bar
              p "yo"
          end
      end
  end 
  
  class Video &lt; ActiveRecord::Base
      include ContentAttribute
  
      # @question = "Specific question"; [should override the generic question]
  end
  </code></pre>