Question

I have a class heirarchy but not single-table inheritence.

ThrowAway < ActiveRecord::Base

and

Junk < ThrowAway

the problem is that all ThrowAway objects have references to a Location. A Location

belongs_to :throw_away, :polymprohic => true

The problem is that if I define

ThrowAway < ActiveRecord::Base
  has_many :locations, :as => :throw_away
end

then even if Junk inherits from it and defines a different table name, the throw_away_type column will always be set to ThrowAway where I actually want it set to Junk.

Now there will be many of these subclasses so there will be Stuff < ThrowAway Rags < ThrowAway etc. I want them all to have a Location without defining a location relationship in each individual class.

Is this possible with rails? Problem is that there is not just location, there are other of these sort of relationships and I’d rather follow some DRY here. I’m assuming I need to create a generator method which will execute on the current object and generate these relationships generators on runtime.

Thanks.

Answer 1

ActiveRecord doesn’t seem to be able to cooperate with model inheritance that’s not STI. Fortunately, that’s probably not what you really want anyway. You probably want a mixin instead.

Create a module that contains all of the functionality that you want your models to share in common, and have all of your models include that module. I would probably put the module in your models directory, but give it an adjective as a name. Some folks might put the module in the lib directory or create a lib directory within the models directory for it.

In your case, you’ll want the association to be defined at the time the module is mixed in, so you’ll need to use a callback. Something like..

module Trashable
  def self.included(base)
    base.send :has_many, :locations, :as => :throw_away
  end
end