RoR错误的未定义方法

我已经看了一段时间,没有看到一个错误。我错过了什么? 当我将一个项目添加到购物车时,出现以下错误

我已经运行了迁移。

.. /Users/computername/.gem/ruby/1.8/gems/activerecord-2.3.5/lib/active\u record/associations/association\u collection.rb:376:in method_missing' /Users/computername/Documents/rails_projects/depot/app/models/cart.rb:5:in 添加产品' /Users/computername/Documents/rails\u projects/depot/app/controllers/line\u items\u controller.rb:46:in `创建'

这是我的创建方法

  def create 
    @cart = find_or_create_cart 
    product = Product.find(params[:product_id]) 
    #@line_item = @cart.line_items.build(:product => product)
    @line_item = @cart.add_product(product.id)
..

我的购物车模型

class Cart < ActiveRecord::Base
  has_many :line_items, :dependent => :destroy

  def add_product(product_id) 
    current_item = line_items.where(:product_id => product_id).first 
    if current_item
      current_item.quantity += 1
    else
      current_item = LineItem.new(:product_id=>product_id)
      line_items << current_item
    end
    current_item
  end
end