ValueError in ManyToManyField _ Django

Question

这就是我工作的模式：

from django.db import models
from django.conf import settings

from products.models import Variation

class CartItem(models.Model):
    cart = models.ForeignKey('Cart')
    items = models.ForeignKey(Variation)
    quantity = models.PositiveIntegerField(default=1)

    def __unicode__(self):
        return self.items.title


class Cart(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, null=True, blank=True)
    items = models.ManyToManyField(Variation, through='CartItem')
    timestamp = models.DateTimeField(auto_now=False, auto_now_add=True)
    updated = models.DateTimeField(auto_now=True, auto_now_add=False)

    def __unicode__(self):
        return (str(self.id))

我以前运行过一次makemigrations和migrate，模型Cart中的items字段是items = models.ManyToManyField(CartItem)

现在，在进行此更改后，我得到以下错误：

ValueError: Cannot alter field carts.Cart.items into carts.Cart.items - they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)

我该怎么解决这个问题？请帮帮忙。

Answer 1

就像错误说的那样，你不能把一个多到多转换成一个外键。您必须将其拆分为两个迁移:首先，完全删除原始字段并运行makemigrations来创建DROP列调用；然后，添加外键并再次运行makemigrations来创建add列。