Multi level categories in Ruby on Rails and Active Record associations

Question

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 field to 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 .

I hope you understand what I mean.

How can reach it?

UPDATE : 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

As I understood, :foreign_key => "parent_id" in has_many :children has to be removed, right?

Answer 1

在此处阅读自我连接模型： http : //guides.rubyonrails.org/association_basics.html#self-joins