在学习<应用Rails进行敏捷Web开发>遇到问题

class Cart
 
 attr_reader :items
 
 def initialize
  @items = []
 end

 def add_product(product)
  current_item = @items.find {|item| item.product == product}
  if current_item
  current_item.increment_quantity
  else
  @items << CartItem.new(product)
  end
 end 
end

class StoreController < ApplicationController
 
 def index
  @products = Product.find_products_for_sale
 end
 
 def add_to_cart
  @cart = find_cart       
  product = Product.find(params[:id])
  @cart.add_product(product)    
 end

 private
 
 def find_cart
  session[:cart] ||= Cart.new
 end 
end

current_item = @items.find {|item| item.product == product}