我正在按照《Agile Web Development with Rails 4》一书学习 Rails，但遇到了这个错误：
NoMethodError in CartsController#destroy
undefined method `name' for nil:NilClass
这是购物车控制器的销毁操作。错误信息引用了该行@cart.destroy if @cart.id == session[:cart_id]:
# DELETE /carts/1
# DELETE /carts/1.json
def destroy
@cart.destroy if @cart.id == session[:cart_id]
session[:cart_id] = nil
respond_to do |format|
format.html { redirect_to store_url,
notice: 'Your cart is currently empty'}
format.json { head :no_content }
end
end
此操作是从“空车”按钮调用的views/carts/shown.html.erb
<%= button_to 'Empty cart', @cart, method: :delete, data: {confirm: 'Are you sure?'} %>
一切工作正常，但在创建添加列的迁移之后price到餐桌上LineItems并修改方法add_product from Carts为了处理这个新列，销毁操作不再起作用。
this is models/cart.rb
class Cart < ActiveRecord::Base
has_many :line_items, dependent: :destroy
def add_product(product_id, product_price)
current_item = line_items.find_by(product_id: product_id)
if current_item
current_item.quantity += 1
else
current_item = line_items.build(product_id: product_id, price: product_price)
end
current_item
end
def total_price
line_items.to_a.sum { |item| item.total_price }
end
end
This is models/line_item.rb:
class LineItem < ActiveRecord::Base
belongs_to :product
belongs_to :cart
def total_price
price * quantity
end
end
这就是动作create from line_items_controller用于将产品添加到购物车：
before_action :set_cart, only: [:create]
...
# POST /line_items
# POST /line_items.json
def create
session[:counter] = 0
product = Product.find(params[:product_id])
@line_item = @cart.add_product(product.id, product.price)
respond_to do |format|
if @line_item.save
format.html { redirect_to @line_item.cart }
format.json { render action: 'show', status: :created, location: @line_item }
else
format.html { render action: 'new' }
format.json { render json: @line_item.errors, status: :unprocessable_entity }
end
end
end
在此操作之前，该方法set_cart从模块CurrentCart叫做：
def set_cart
@cart = Cart.find(session[:cart_id])
rescue ActiveRecord::RecordNotFound
@cart = Cart.create
session[:cart_id] = @cart.id
end
当我运行控制器测试时，没有出现任何故障。这是对动作的测试destroy来自购物车：
test "should destroy cart" do
assert_difference('Cart.count', -1) do
session[:cart_id] = @cart.id
delete :destroy, id: @cart
end
assert_redirected_to store_path
end
我不明白这个错误的原因是什么，我需要你的帮助。
edit
这是服务器的输出：
Started DELETE "/carts/15" for 127.0.0.1 at 2015-02-16 12:23:43 -0500
Processing by CartsController#destroy as HTML
Parameters: {"authenticity_token"=>"hLsBQ0fR5b9M2lfxMc5McU1+dqtnZ2OKABRZV79vdHo=", "id"=>"15"}
Cart Load (0.2ms) SELECT "carts".* FROM "carts" WHERE "carts"."id" = ? LIMIT 1 [["id", "15"]]
(0.2ms) begin transaction
LineItem Load (0.9ms) SELECT "line_items".* FROM "line_items" WHERE "line_items"."cart_id" = ? [["cart_id", 15]]
SQL (0.5ms) DELETE FROM "line_items" WHERE "line_items"."id" = ? [["id", 91]]
(0.2ms) rollback transaction
Completed 500 Internal Server Error in 73ms
NoMethodError (undefined method `name' for nil:NilClass):
app/controllers/carts_controller.rb:58:in `destroy'
谢谢 ：）
