for the record

Ok, I just want to set this one in stone.

Rails’ polymorphic associations.

1. Can this object belong to more than 1 type of object

   
   class Car < ActiveRecord::Base
   end
   
   class House < ActiveRecord::Base
   end
   
   class LineItem < ActiveRecord::Base
   
     belongs_to :salable, :polymorphic => true
   
   end
   

   Here’s a domain model from an app in which you can purchase both cars and houses. They are both salable products that will appear on a line item in an order. This is a many-to-1 relationship from a LineItem to its salable product.
2. Can this object belong to more than 1 type of object and more than 1 object

   
   class User < ActiveRecord::Base
   
     include Groupable
   
   end
   
   class Account < ActiveRecord::Base
   
     include Groupable
   
   end
   
   class Membership < ActiveRecord::Base
   
     belongs_to :groupable, :polymorphic => true
     belongs_to :group
   
   end
   
   class Group < ActiveRecord::Base
   
     has_many :memberships
   
   end
   
   module Groupable
   
    def self.included(clazz)
       clazz.class_eval
         has_many :memberships, :as => :groupable
       end
     end
   
   end
   

   Here we have a many-to-many between groupable objects and groups. Since a Group can contain members of different types it needs to be polymorphic. However, in this example a single group can have multiple members all of different types. Like any many-to-many we need a table between the 2 other tables. Since habtm doesn’t support polymorphic associations we have to go with a join model. So we introduced Membership and made it have a polymorphic relationship.

   It’d be sweet if we didnt need this join model because I can’t stand unnecessary classes, something like:

   
   class User < ActiveRecord::Base
   
     include Groupable
   
   end
   
   class Account < ActiveRecord::Base
   
     include Groupable
   
   end
   
   class Group < ActiveRecord::Base
   
     has_and_belongs_to_many :groupables
   
   end
   
   module Groupable
   
     def self.included(clazz)
       clazz.class_eval
         has_and_belongs_to_many :groups, :as => :groupable
       end
     end
   
   end
   

   db schema:

   
     groupables (groupable_id, groupable_type, group_id)
   
   

   That’s some made up syntax. I’ll say if you have an :as parameter to #has_and_belongs_to_many then Rails will look for a table named after the polymorphic interface, in this case groupables, and 2 columns groupable_id and groupable_type. Since the #has_and_belongs_to_many call in the Group model doesn’t include an :as parameter, Rails will, like in a normal #has_and_belongs_to_many call, look for a table named after the association, in this case groupables, and a foreign key in that table referencing this model, in this case group_id.