class Foo
  def self.to_mongo(value)
    # convert value to a mongo safe data type
  end

  def self.from_mongo(value)
    # convert value from a mongo safe data type to your custom data type
  end
end

class Thing
  include MongoMapper::Document
  key :name, Foo                #Awesome
  key :count, Integer
  key :slug, String
end