gistfile1.rb

#!/usr/bin/env ruby -Ku

# encoding: utf-8

require 'rubygems'
require 'dm-core'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite3::memory:')

class User
  include DataMapper::Resource

  property :id, Serial

  has n, :ownerships, :inverse => :owner
  has n, :games, :through => :ownerships
end

class Game
  include DataMapper::Resource

  property :id, Serial

  has n, :ownerships
  has n, :owners, 'User', :through => :ownerships  # FIXME: explicit "User" will not always be necessary
end

class Ownership
  include DataMapper::Resource

  property :game_id,  Integer, :key => true, :min => 1
  property :owner_id, Integer, :key => true, :min => 1

  belongs_to :game
  belongs_to :owner, 'User'
end

DataMapper.auto_migrate!

puts '-' * 80
User.create(:games  => [ {} ])

puts '-' * 80
Game.create(:owners => [ {} ])