参照敏捷开发 4 中的代码。定义了购物车，我另外加了个devise来验证用户，可是到我退出再登录的时候，购物车就自动清空了。不能保存上次的添加进购物车的内容。 购物车已经存进数据库了。 现在就不知道怎么购物车跟当前的用户关联起来。

这个是加入的购物车的定义的内容

class CartsController < ApplicationController

 before_filter :extract_cart

 def create
   @product = Product.find(params[:product_id])
   @cart.add(@product, @product.title)
   redirect_to cart_path
 end 

 private

  def extract_cart
      cart_id = session[:cart_id]
    @cart = session[:cart_id] ? Cart.find(cart_id) : Cart.create
    session[:cart_id] = @cart.id
  end     

这个要怎么让用户加入购物车的时候抓到用户的 ID，让下次登录的时候也能看到购物车的内容

这个是CartItems表中的字段

CartItem Load (0.4ms)  SELECT "cart_items".* FROM "cart_items" 
 => [#<CartItem id: 1, owner_id: 7, owner_type: "Cart", quantity: 6, item_id: 1, item_type: "Product", price: 0.0, created_at: "2013-09-03 06:09:06", updated_at: "2013-09-03 06:30:04", user_id: nil>, #<CartItem id: 2, owner_id: 7, owner_type: "Cart", quantity: 1, item_id: 3, item_type: "Product", price: 0.0, created_at: "2013-09-03 06:14:47", updated_at: "2013-09-03 06:14:47", user_id: nil>, #<CartItem id: 3, owner_id: 7, owner_type: "Cart", quantity: 3, item_id: 4, item_type: "Product", price: 123.0, created_at: "2013-09-03 06:15:25", updated_at: "2013-09-03 06:32:13", user_id: nil>, #<CartItem id: 4, owner_id: 7, owner_type: "Cart", quantity: 1, item_id: 6, item_type: "Product", price: 333.0, created_at: "2013-09-03 06:15:41", updated_at: "2013-09-03 06:15:41", user_id: nil>, #<CartItem id: 5, owner_id: 7, owner_type: "Cart", quantity: 3, item_id: 5, item_type: "Product", price: 222.0, created_at: "2013-09-03 06:15:45", updated_at: "2013-09-03 06:18:00", user_id: nil>, #<CartItem id: 6, owner_id: 8, owner_type: "Cart", quantity: 1, item_id: 1, item_type: "Product", price: 0.0, created_at: "2013-09-03 06:36:00", updated_at: "2013-09-03 06:36:00", user_id: nil>, #<CartItem id: 7, owner_id: 8, owner_type: "Cart", quantity: 1, item_id: 2, item_type: "Product", price: nil, created_at: "2013-09-03 06:36:05", updated_at: "2013-09-03 06:36:05", user_id: nil>, #<CartItem id: 8, owner_id: 8, owner_type: "Cart", quantity: 2, item_id: 3, item_type: "Product", price: 0.0, created_at: "2013-09-03 06:36:10", updated_at: "2013-09-03 06:36:10", user_id: nil>]

这个是要怎么解决，还希望各位大牛指导