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