I have users, posts, and comments. Posts belong to users. Users have many posts.

Now where I'm confused is that users are supposed to be able to comment on others' posts.
How do I set up this association?

Should I write users have many comments, posts have many comments, comments belong to posts and users? or user has many comments through posts?
Also what columns should the model table have? Does it need user\_id, post\_id, content?