Many To Many Relations

products
id	`int`
name	`string`

categories
id	`int`
name	`string`

categories_products
category_id	`int`
product_id	`int`

class Category < ActiveRecord::Base
  has_and_belongs_to_many :products
end

class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

#You can now use

acategory.products #get all products with acategory
aproduct.categories #yes, it pluralizes category correctly