include both

models/comment.rb

@@ -47,7 +47,7 @@ class Comment < Content
     if params[:recursive]
       self.class.hash_tree(subtree(sort: sort_by_parent_and_time)).first
     else
-      as_document.slice(*%w[body course_id endorsed _id updated_at]).
+      as_document.slice(*%w[body course_id endorsed _id created_at updated_at]).
                   merge("user_id" => author.id).
                   merge("votes" => votes.slice(*%w[count up_count down_count point]))
     end

models/comment_thread.rb

@@ -37,7 +37,7 @@ class CommentThread < Content
   end
 
   def to_hash(params={})
-    doc = as_document.slice(*%w[title body course_id _id updated_at]).
+    doc = as_document.slice(*%w[title body course_id _id created_at updated_at]).
                       merge("user_id" => (author.id if author)).
                       merge("votes" => votes.slice(*%w[count up_count down_count point]))
     if params[:recursive]