Gemfile.rb

source 'http://gemcutter.org'

gem 'activesupport',     '~> 3.0.0.beta3', :require => 'active_support'

gem 'dm-core',           '~> 0.10.3', :git => 'git://github.com/datamapper/dm-core.git'
gem 'dm-types',          '~> 0.10.3', :git => 'git://github.com/datamapper/dm-types.git'
gem 'dm-validations',    '~> 0.10.3', :git => 'git://github.com/datamapper/dm-validations.git'
gem 'dm-migrations',     '~> 0.10.3', :git => 'git://github.com/datamapper/dm-migrations.git'

gem 'dm-do-adapter', '~> 0.10.3', :git => 'git://github.com/datamapper/dm-do-adapter.git'
gem 'dm-mysql-adapter', :git => 'git://github.com/datamapper/dm-mysql-adapter.git'

gem 'data_objects',      '~> 0.10.1'
gem 'do_mysql', :git => 'git://github.com/datamapper/do.git'

gistfile2.rb

require 'rubygems'
require 'bundler'

Bundler.setup
Bundler.require :default

class A
  include DataMapper::Resource

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

  before :valid? do
    puts "foo"
  end
end

class B < A
  before :valid? do
    puts "bar"
  end
end