Let users reply to comments

Previously: comments only replied to posts
Problem: reddit has really neat threaded conversations of comments replying to comments replying to comments, replying to (etc)
Solution: 
-make comments belong to a polymorphic "parent" which they are responding to (either posts or other comments)
-make the _comment partial recursive, so that a comment displays all its replies just below it in a nested list
I want to distinguish between top-level comments ("replies" to the post) and all comments that a post has (the set of all comments and threads on a post). is this a good way to do that?