Question

I have a legacy database that I’m working on getting ActiveRecord to work with. I’ve run into a problem with join tables. I have the following:

class TvShow < ActiveRecord::Base

  set_table_name "tvshow"
  set_primary_key "idShow"

end

class Episode < ActiveRecord::Base
  set_table_name "episode"
  set_primary_key "idEpisode"
end

And then I have a table called tvshowlinkepisode that has 2 fields: idShow, idEpisode So I have 2 tables and a join between them (so a many to many relationship), however the join uses non-standard foreign keys. My first thought was to create a model called TvShowEpisodeLink but there isn’t a primary key. The idea was that since the foreign keys are non-standard I could use the set_foreign_key and have some control. Ultimately I want to say something like TvShow.find(:last).episodes or Episode.find(:last).tv_show. How do I get there?

Answer 1

I believe you can be slightly more elegant than Alvaro’s answer using options to has_and_belongs_to_many, though his answer is perfectly fine and will result in fairly identical functionality for any clients of your class.

class TvShow < ActiveRecord::Base

  set_table_name "tvshow"
  set_primary_key "idShow"
  has_and_belong_to_many :episodes, 
                         :join_table => "tvshowlinkepisode", 
                         :foreign_key => "idShow",
                         :association_foreign_key => "idEpisode"

end

class Episode < ActiveRecord::Base
  set_table_name "episode"
  set_primary_key "idEpisode"
  has_and_belongs_to_many :tv_shows,
                          :join_table => "tvshowlinkepisode",
                          :foreign_key => "idEpisode",
                          :association_foreign_key => "idShow"
end

Note that the :foreign_key option specifies which column is the id for the class on “this side” of the link, while :association_foreign_key specifies the column that is the id for the class on the “other side” of the link.

Contrast this with Alvaro’s answer, this pattern should avoid instantiation of any unnecessary objects to represent the link.