gistfile1.rb

require 'rubygems'
gem 'dm-core', '0.10.0'
require 'dm-core'
 
DataMapper.setup(:default, 'postgres://tpitale@localhost/test_through_many')

class User
  include DataMapper::Resource
  property :id, Serial
  has n, :sales
  has n, :sale_items, :through => :sales
  has n, :items, :through => :sale_items
end

class Sale
  include DataMapper::Resource
  property :id, Serial

  belongs_to :user
  has n, :sale_items
  has n, :items, :through => :sale_items
end

class SaleItem
  include DataMapper::Resource
  property :id, Serial
  
  belongs_to :sale
  belongs_to :item
end

class Item
  include DataMapper::Resource
  property :id, Serial
  
  has n, :sale_items
end

DataMapper.auto_migrate!

user = User.create
sale = user.sales.create

10.times {sale.items.create}

sale.reload
user.reload

puts sale.items.inspect
puts user.items.inspect