vendor/rails/activerecord/test/models/tag.rb

   1 class Tag < ActiveRecord::Base
   2   has_many :taggings
   3   has_many :taggables, :through => :taggings
   4   has_one  :tagging
   5
   6   has_many :tagged_posts, :through => :taggings, :source => :taggable, :source_type => 'Post'
   7 end