gistfile1.rb

class Person < ActiveRecord::Base
  has_many :orders
  has_many :lines, :through => :products
  has_many :products, :through => :lines
end

class Order < ActiveRecord::Base
  has_many :lines
  has_many :products, :through => :lines
end

class Line < ActiveRecordd::Base
  belongs_to :product
end

class Product < ActiveRecord::Base
  has_many :lines
end