gistfile1.rb

class Car
  include MongoMapper::Document
  key :make, String
  key :_type, String
end

class Honda < Car
  key :honda_thing, String
end

class Toyota < Car
  key :recalled, Boolean
end

# script/console:
# >> Car.all
# => [#<Honda _id: 4b75b5d7844de47805000001, _type: "Honda", recalled: nil, make: nil, honda_thing: "honda!">, #<Toyota _id: 4b75b5e0844de47805000002, _type: "Toyota", recalled: false, make: nil, honda_thing: nil>]

# mongo:
# > db.cars.find();
# { "_id" : ObjectId("4b75b5d7844de47805000001"), "_type" : "Honda", "make" : null, "recalled" : null, "honda_thing" : "honda!" }
# { "_id" : ObjectId("4b75b5e0844de47805000002"), "_type" : "Toyota", "make" : null, "recalled" : false, "honda_thing" : null }