many-to-many on RoR

article.rb

class Article 
  has_many :categories, :through => :category_articles 
  has_many :category_articles 
end

category.rb

class Category 
  has_many :articles, :through => :category_articles 
  has_many :category_articles 
end 

category_article.rb

class CategoryArticle
  belongs_to :article
  belongs_to :category, :counter_cache => true 
end