return commentable_id of threads

models/comment_thread.rb

@@ -69,7 +69,7 @@ class CommentThread < Content
   end
 
   def to_hash(params={})
-    doc = as_document.slice(*%w[title body course_id created_at updated_at])
+    doc = as_document.slice(*%w[title body course_id commentable_id created_at updated_at])
                       .merge("id" => _id)
                       .merge("user_id" => (author.id if author))
                       .merge("votes" => votes.slice(*%w[count up_count down_count point]))

spec/api/comment_thread_spec.rb

@@ -13,6 +13,7 @@ describe "app" do
         thread.body.should == response_thread["body"]
         thread.course_id.should == response_thread["course_id"]
         thread.votes_point.should == response_thread["votes"]["point"]
+        thread.commentable_id.should == response_thread["commentable_id"]
         response_thread["children"].should be_nil
       end
       it "get information of a single comment thread with its comments" do