gistfile1.rb

#!/usr/bin/env ruby -Ku

# encoding: utf-8

require 'pp'
require 'rubygems'
require 'dm-core'

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

class User
  include DataMapper::Resource

  property :id,   Serial
  property :name, String, :length => 1..100, :required => true

  has n, :roles, :through => Resource
end

class Role
  include DataMapper::Resource

  property :id,   Serial
  property :name, String, :length => 1..20, :required => true

  has n, :users, :through => Resource
end

DataMapper.auto_migrate!

User.create(:name => 'Dan Kubb', :roles => [ { :name => 'Administrator' } ])
User.create(:name => 'John Doe', :roles => [ { :name => 'User'          } ])
User.create(:name => 'Jane Doe', :roles => [ { :name => 'User'          } ])

puts '-' * 80

pp User.all(:roles => { :name => 'Administrator' })
pp User.all(:roles => { :name => 'User'          })