files_with_childs.rb

#!/usr/bin/env ruby
#
# encoding: utf-8

require 'dm-sqlite-adapter'
require 'dm-migrations'
require 'dm-validations'

DataMapper.setup :default, "sqlite::memory:"

module App
  class File
    include DataMapper::Resource

    property :id,   Serial
    property :type, Discriminator
    property :name, String

    belongs_to :folder, :required => false
  end

  class Folder < File
    has n, :files
  end
end

DataMapper.finalize
DataMapper.auto_migrate!

file   = App::File.new(:name => "foo.txt")
folder = App::Folder.create(:name => "Docs", :files => [file])

puts folder.files.inspect