extra_property.rb

require 'rubygems'
require 'dm-core'

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

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

class User
  include DataMapper::Resource

  property :id, Serial

  has n, :sales
  has n, :sale_items, :through => :sales
  # has n, :items, :through => :sale_items
end

Sale.auto_migrate!
# Sale is fine here
puts Sale.new.inspect

DataMapper.auto_migrate!

# Sale now has sale_item_id
puts Sale.new.inspect