Best way to implement Categories and Subcategories with flexible depth in Rails?

I have a Category and Subcategory models 开发者_高级运维in my project. I would like to have many sub-levels in a flexible way. I thought making a self referential "parent" foreign key but I'm not quite sure how to do it. Any ideas? Thanks!

Cat1
    Sub1
        SubSub1
        SubSub2
    Sub2
Cat2
    Sub1
Cat3
    Sub1
    Sub2
        SubSub1

Try the acts_as_tree plug-in

Awesome_nested_set is the best solution, imo, for these types of structures in rails. http://github.com/collectiveidea/awesome_nested_set