Cart.cart_id : NULL约束失败：

Question

我正试图创建一个电子商务网站。我需要一个‘购物车’应用程序，将产生一个购物车对象，将持有项目，用户想要购买。

但是，当向购物车添加项时，我得到：

IntegrityError at /cart/add/3/ NULL约束失败: Cart.cart_id

Cart对象将有一个ID，这将是用户的会话ID。

这是将获得会话ID的函数：

def _card_id(request):
    cart = request.session.session_key
    if not cart:
        cart = request.session.create()
        return cart

views.py

from django.shortcuts import render, redirect
from shop.models import Product
from .models import Cart, CartItem
from django.core.exceptions import ObjectDoesNotExist


# Create your views here.

def _card_id(request):
    cart = request.session.session_key
    if not cart:
        cart = request.session.create()
        return cart


def add_cart(request, product_id):
    product = Product.objects.get(id = product_id)
    try:
        cart = Cart.objects.get(cart_id = _card_id(request))
    except Cart.DoesNotExist:
        cart = Cart.objects.create(
            cart_id = _card_id(request)
        )

        cart.save()
    try:
        cart_item = CartItem.objects.get(product = product, cart = cart)
        cart_item.quantity += 1
        cart_item.save()
    except CartItem.DoesNotExist:
        cart_item = CartItem.objects.create(
            product = product,
            quantity= 1,
            cart = cart,
        )
        cart_item.save()
    return redirect('cart:cart_detail')


def cart_detail(request, total = 0, counter = 0, cart_items = None):
    try:
        cart = Cart.objects.get(cart_id = _card_id(request))
        cart_items =  CartItem.objects.filter(cart = cart, active=True)

        for cart_item in cart_items:
            total += (cart_item.product.price * cart_item.quantity)
            counter += cart_item.quantity
    except ObjectDoesNotExist:
        pass

    return render(request, 'cart.html', dict(cart_items = cart_items, total = total, counter = counter))

models.py:

from django.db import models
from shop.models import Product

# Create your models here.

class Cart(models.Model):
    cart_id = models.CharField(max_length=250, blank=True)
    date_added = models.DateField(auto_now_add=True)

    class Meta:
        db_table = 'Cart'
        ordering = ['date_added']

    def __str__(self):
        return self.cart_id


class CartItem(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    cart = models.ForeignKey(Cart, on_delete=models.CASCADE)
    quantity = models.IntegerField()
    active = models.BooleanField(default=True)
    class Meta:
        db_table = 'CartItem'

    def sub_total(self):
        return self.product.price * self.quantity

    def __str__(self):
        return self.product

urls:

from django.urls import path
from . import views

app_name = 'cart'

urlpatterns = [
    path('add/<int:product_id>/', views.add_cart, name = 'add_cart'),
    path('', views.cart_detail, name = 'cart_detail'),
]

完全错误：

IntegrityError at /cart/add/3/
NOT NULL constraint failed: Cart.cart_id
Request Method: GET
Request URL:    http://127.0.0.1:8000/cart/add/3/
Django Version: 2.1.3
Exception Type: IntegrityError
Exception Value:    
NOT NULL constraint failed: Cart.cart_id
Exception Location: /home/ogonzales/Escritorio/projects_envs/perfectcushion_env/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py in execute, line 296
Python Executable:  /home/ogonzales/Escritorio/projects_envs/perfectcushion_env/bin/python
Python Version: 3.6.7
Python Path:    
['/home/ogonzales/Escritorio/web_proyects/perfectcushion',
 '/home/ogonzales/Escritorio/web_proyects/perfectcushion',
 '/usr/lib/python36.zip',
 '/usr/lib/python3.6',
 '/usr/lib/python3.6/lib-dynload',
 '/home/ogonzales/Escritorio/projects_envs/perfectcushion_env/lib/python3.6/site-packages',
 '/home/ogonzales/Escritorio/projects_envs/perfectcushion_env/lib/python3.6/site-packages/setuptools-39.1.0-py3.6.egg',
 '/home/ogonzales/Escritorio/projects_envs/perfectcushion_env/lib/python3.6/site-packages/pip-10.0.1-py3.6.egg',
 '/home/ogonzales/Escritorio/pycharm/helpers/pycharm_matplotlib_backend']
Server time:    Sun, 2 Dec 2018 15:21:38 +0000

Answer 1

你需要在这里换衣服：

def _card_id(request):
    cart = request.session.session_key
    if not cart:
        request.session.create()  # it does not return anything. that is why `cart = request.session.create()` will not work
        cart = request.session.session_key
    return cart  # Ultimately return cart

Answer 2

我认为您创建了该对象，并且在_cart_id函数能够触发之前的一小段时间内，该对象没有cart_id的值，这是不允许您指定模型的方式的。也许可以尝试将null=True添加到购物车模型中的cart_id字段中。这样，在_cart_id函数能够给出值之前，允许字段在短时间内为空。

cart_id = models.CharField(max_length=250, null=True, blank=True)

Answer 3

试着改变这个

cart = models.ForeignKey(Cart, on_delete=models.CASCADE)

至

cart = models.ForeignKey(Cart, on_delete=models.CASCADE,null=True)

对我来说，这就是问题所在