has_and_belongs_to_many in Ruby on Rails
http://wiki.rubyonrails.org/rails/pages/has_and_belongs_to_many
habtm いろいろはまりみち@_@
Jeff Hicks
http://jrhicks.net/96
Encytemedia: Storing additional data on join tables with Rails
http://encytemedia.com/blog/articles/2005/06/15/storing-additional-data-on-join-tables-with-rails