Auto-association cascading

In my Rails learning this site has been very fruitful. My question has to do with Rails + PostgreSQL:

I learned the PostgreSQL is to make self-association and plan to use it in the following situation;

1- I am doing a forum that has categories.

2- When the category_id be |= (different) null / NILL mean he is in the category that was made to the association.

And so on may have several subcategories cascade, and I intend to do the same thing with topics and replies of topics.

I wonder how I associate it in the model and how to do a search in cascade form in the rails?

Thanks,