问Ajax更新失败与Rails 4的敏捷Web开发

EN

Started POST "/line_items?product_id=1" for 127.0.0.1 at 2014-07-08 17:47:36 +0100
Processing by LineItemsController#create as JS
  Parameters: {"authenticity_token"=>"O+QFilrdqd/AsAoJGHzayWphHT8PDBEBqnjFIAXqQ7Y=", "product_id"=>"1"}
  Cart Load (0.1ms)  SELECT  "carts".* FROM "carts"  WHERE "carts"."id" = ? LIMIT 1  [["id", 6]]
  Product Load (0.1ms)  SELECT  "products".* FROM "products"  WHERE "products"."id" = ? LIMIT 1  [["id", 1]]
  LineItem Load (0.1ms)  SELECT  "line_items".* FROM "line_items"  WHERE "line_items"."cart_id" = ? AND "line_items"."product_id" = 1 LIMIT 1  [["cart_id", 6]]
   (0.1ms)  begin transaction
  SQL (0.3ms)  UPDATE "line_items" SET "quantity" = ?, "updated_at" = ? WHERE "line_items"."id" = 10  [["quantity", 7], ["updated_at", "2014-07-08 16:47:36.666376"]]
   (1.5ms)  commit transaction
  LineItem Load (0.1ms)  SELECT "line_items".* FROM "line_items"  WHERE "line_items"."cart_id" = ?  [["cart_id", 6]]
  Product Load (0.1ms)  SELECT  "products".* FROM "products"  WHERE "products"."id" = ? LIMIT 1  [["id", 1]]
  Product Load (0.1ms)  SELECT  "products".* FROM "products"  WHERE "products"."id" = ? LIMIT 1  [["id", 2]]
  Rendered line_items/_line_item.html.erb (1.9ms)
  Rendered carts/_cart.html.erb (4.3ms)
  Rendered line_items/create.js.erb (5.4ms)
Completed 200 OK in 14ms (Views: 7.3ms | ActiveRecord: 2.4ms)

<h2>Your cart<h2>
<table>
  <%= render(cart.line_items) %>

  <tr class="total_line">
    <td colspan="2">Total</td>
    <td class="total_cell"><%= quid(cart.total_price) %></td>
  </tr>
</table>
<%= button_to 'Empty cart', cart, method: :delete, data: { confirm: 'Are you sure?' }%>

<tr>
  <td><%= line_item.quantity %>&times;</td>
  <td><%= line_item.product.title %></td>
  <td class="item_price"><%= quid(line_item.total_price) %></td>
</tr>

$('#cart').html('<%= escape_javascript render(@cart) %>');
$('#cart').html(alert("alert"));