Ruby on rails Rails未定义方法错误突然出现

我正在使用购物车制作一个应用程序，购物车已经运行了数周，直到今天，我随机发现以下错误：

undefined method `title' for nil:NilClass

Extracted source (around line #20):

            <%= link_to product.title, product %>

                <p><%= number_to_currency(product.price, :unit => '$') %></p>
                <p>Quantity: <%= quantity %></p>

---

您需要避免在视图中执行查询，但是，如果您想保留该

产品。按\u id查找\u

，则可以在那里添加一个guard类：

<% @cart.each do | id, quantity | %>
 <% product = Product.find_by_id(id)  %>
  <% if product %>
    <li>

        <%= link_to product.title, product %>

        <p><%= number_to_currency(product.price, :unit => '$') %></p>
        <p>Quantity: <%= quantity %></p>

    </li>
    <% total += quantity * product.price %>
   <% end %>
<% end %>

“$”）%>

数量：

<div class="shoping-cart">
<h1>Your Cart</h1>

<% if  @cart.empty? %>
    <p>Your cart is currently empty</p>
<% else %>
    <%= link_to 'Empty Cart', cart_clear_path %>
<% end %>

<br><br><br>

<% total = 0 %>
<div class="list">
<ul>
<% @cart.each do | id, quantity | %>
    <% product = Product.find_by_id(id)  %>

        <li>

            <%= link_to product.title, product %>

            <p><%= number_to_currency(product.price, :unit => '$') %></p>
            <p>Quantity: <%= quantity %></p>

        </li>
        <% total += quantity * product.price %>

<% end %>

<br><br><br>


<p><p><%= number_to_currency(total, :unit => '$') %></p></p>
</ul>
</div>
<% link_to 'pay now',  new_charge_path %>

</div>

 get '/cart' => 'cart#index'
  get '/cart/clear' => 'cart#clearCart'
  get '/cart/:id' => 'cart#add'

<% @cart.each do | id, quantity | %>
 <% product = Product.find_by_id(id)  %>
  <% if product %>
    <li>

        <%= link_to product.title, product %>

        <p><%= number_to_currency(product.price, :unit => '$') %></p>
        <p>Quantity: <%= quantity %></p>

    </li>
    <% total += quantity * product.price %>
   <% end %>
<% end %>