gistfile1.txt

irb(main):055:0> o = Order.last
  Order Load (0.4ms)  SELECT "orders".* FROM "orders" ORDER BY "orders"."id" DESC LIMIT 1
=> #<Order id: 143, customer_id: 13, session_id: "5c7f3b0268022483ed1c87ff9fb5304a", state: nil, discount: #<BigDecimal:7fe419d863f0,'0.0',9(18)>, balance: #<BigDecimal:7fe419d86328,'0.0',9(18)>, notes: nil, shipping_method: nil, shipping_details: nil, created_at: "2012-09-16 17:27:39", updated_at: "2012-09-16 21:09:56">
irb(main):056:0> oi = o.order_items.includes(item: :photos)
  OrderItem Load (0.4ms)  SELECT "order_items".* FROM "order_items" WHERE "order_items"."order_id" = 143
  Item Load (0.2ms)  SELECT "items".* FROM "items" WHERE "items"."id" IN (227)
  Photo Load (0.1ms)  SELECT "photos".* FROM "photos" WHERE "photos"."item_id" IN (227)
=> [#<OrderItem id: 302, order_id: 143, item_id: 227, item_price: #<BigDecimal:7fe4198f21b8,'0.5E2',9(18)>, quantity: 1, size: 95, type: "rent", starts_on: "2012-09-17", ends_on: "2012-09-18", created_at: "2012-09-16 20:03:33", updated_at: "2012-09-16 21:09:56">]
irb(main):057:0> o.order_items
  OrderItem Load (0.4ms)  SELECT "order_items".* FROM "order_items" WHERE "order_items"."order_id" = 143
=> [#<OrderItem id: 302, order_id: 143, item_id: 227, item_price: #<BigDecimal:7fe4199014b0,'0.5E2',9(18)>, quantity: 1, size: 95, type: "rent", starts_on: "2012-09-17", ends_on: "2012-09-18", created_at: "2012-09-16 20:03:33", updated_at: "2012-09-16 21:09:56">]
irb(main):058:0> o.total
  Item Load (0.4ms)  SELECT "items".* FROM "items" WHERE "items"."id" = 227 LIMIT 1
=> #<BigDecimal:7fe419907310,'0.5E2',9(36)>

item.rb

class Item < ActiveRecord::Base
  has_many :order_items
  has_many :orders, through: :order_items

  attr_accessible :rent_price, :sale_price
end

order.rb

class Order < ActiveRecord::Base
  has_many :order_items, dependent: :destroy

  def total
    order_items.map(&:price).reduce(:+)
  end
end

order_item.rb

class OrderItem < ActiveRecord::Base
  belongs_to :item
  belongs_to :order

  attr_accessible :type, :quantity

  def item_price
    item.send("#{type}_price")
  end

  def price
    quantity * item_price
  end
end