diff --git a/lib/ext/entities.rb b/lib/ext/entities.rb
index 4881f51..502a353 100644
--- a/lib/ext/entities.rb
+++ b/lib/ext/entities.rb
@@ -2,12 +2,19 @@ module Noosfero
   module API
     module Entities
 
-      class ArticleBase < Entity
-        expose :ranking_position do |article, options|
-          article.kind_of?(ProposalsDiscussionPlugin::Proposal) && article.ranking_item.present? ? article.ranking_item.position : nil
+      def self.included(base)
+        base.extend(ClassMethods)
+      end
+
+      module ClassMethods
+        class ArticleBase < Entity
+          expose :ranking_position do |article, options|
+            article.kind_of?(ProposalsDiscussionPlugin::Proposal) && article.ranking_item.present? ? article.ranking_item.position : nil
+          end
         end
       end
 
     end
   end
+
 end
--
libgit2 0.21.2