gistfile1.rb

class ArbitraryModel < DataMapper::Type

  primitive String

  def self.dump(value, property)
    "#{value.class}/#{value.id}" unless value.nil?
  end

  def self.load(value, property)
    case value
      when String
        type, id = *value.split('/')
        if (type = type.constantize) && type.is_a?(DataMapper::Model)
          type.get(id)
        else
          raise ArgumentError.new('Only DataMapper models may be assigned!')
        end
      else
        value
    end
  end

  def self.typecast(value, property)
    load(value, property)
  end

end