gistfile1.rb

class Order < ActiveRecord::Base
  has_many :order_products
  validates_associated :order_products # This checks all the order_product instances are valid
end

class OrderProduct < ActiveRecord::Base
  belongs_to :order
  belongs_to :product
  validates :order, :product, :presence => true
end

class Product < ActiveRecord::Base
  has_many :order_products
  validates :name, :description, :price, :presence => true
end