## box.rb
class Box
  include DataMapper::Resource
  property :id, Serial
  has n, :broken_toothpicks
end

## toothpick.rb
class Toothpick
  include DataMapper::Resource
  property :id, Serial
  property :type, Discriminator
  belongs_to :box
end

## broken_toothpick.rb
class BrokenToothpick < Toothpick
  property :pieces, Integer, :default => 2
end

## What happens?

ruby-1.8.7-p330 :001 > b = Box.create
 => #<Box @id=2>
ruby-1.8.7-p330 :002 > b.broken_toothpicks
 => []
ruby-1.8.7-p330 :003 > b.broken_toothpicks.create
 => #<BrokenToothpick @id=nil @type=BrokenToothpick @pieces=2 @box_id=nil>
ruby-1.8.7-p330 :004 >