Polymorphic associations: finding object types

Hi there

I'm developing an application which makes use of RoR's polymorphic
associations to associate several components with a single product,
like so:

class Product < ActiveRecord::Base
  has_many :line_items, :before_add => :validate_addition
end

class LineItem < ActiveRecord::Base
  belongs_to :product
  belongs_to :resource, :polymorphic => true
end

class Circuit < ActiveRecord::Base
  has_one :line_item, :as => :resource
end

class UserAccount < ActiveRecord::Base
  has_one :line_item, :as => :resource
end

I'd like to be able to obtain a list of the objects which act as a
resource for use in the interface. I could do this from the database
like so: