gistfile1.txt

class Order
  has_many :foods, :through => :line_items
  has_many :merchandise_items, :through => :line_items
  has_many :line_items

  def total
    line_items.map(&:price).sum
  end
end

class LineItem
  belongs_to :food
  belongs_to :merchandise_item

  def price
    food ? 100 : merchandise_item.price
  end
end

receipt = Receipt.new
receipt.foods << Food.find("sandwich")

receipt.foods.first  # This is #<Food id: "sandwich"> as it should be
receipt.line_items   # this is unfortunately empty until we save the order!
receipt.total        # this is zero, because there are no invoice items!