Categorías multinivel en Ruby on Rails y asociaciones Active Record

Ineed to create multilevel categories in Ruby on Rails. So I create a model Category which has title and description and has many articles.

class Category
  has_many :articles
end

Then I need to add parent_id campo para Category model. This field must be either null (if it's a parent category) or has some id (if it's a child category). Obviously, to select any parent category it has to select Select * from Categories where parent_id=null.

Espero entiendas lo que quiero decir.

How can reach it?

ACTUALIZACIÓN: Thank you for your suggestion. Here is what I have

class Category < ActiveRecord::Base
  belongs_to :parent, :class_name => "Category", :foreign_key => "parent_id"
  has_many :children,  :class_name => "Category", :foreign_key => "parent_id"
  attr_accessible :description, :title
end

Como entendí, :foreign_key => "parent_id" in has_many :children has to be removed, right?