DataMapper association woes

gistfile1.rb

module TruckPricer
  class Engine
    include DataMapper::Resource
    property :id, Serial, :key => true
    property :name, String, :required => true
    property :vin_string, String
    belongs_to :price, :required => false
  end
end


module TruckPricer
  class TruckModel
    include DataMapper::Resource
    property :id, Serial, :key => true
    property :name, String, :required => true
    property :vin_string, String
    belongs_to :price, :required => false
  end
end

#I want this model to contain the ids of the TruckModel and Engine models


require 'dm-serializer'

module TruckPricer
  class Price
    include DataMapper::Resource
    property :id, Serial, :key => true
    property :price, Float
    has 1, :truck_model
    has 1, :engine
  end
end

#Would like to be able to look up Price by the TruckModel and Engine associated with it like so:
TruckPricer::Price.first(:truck_model_id => 1, :engine_id => 1)
#This fails because Price doesn't have truck_model_id or engine_id attributes.