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

   1 class Citation < ActiveRecord::Base
   2   belongs_to :reference_of, :class_name => "Book", :foreign_key => :book2_id
   3
   4   belongs_to :book1, :class_name => "Book", :foreign_key => :book1_id
   5   belongs_to :book2, :class_name => "Book", :foreign_key => :book2_id
   6 end