使用Djang时迁移到模型时获取TypeError

Question

我目前正在从ericmatthes的《Python速成课程》一书中学习Python。我正在尝试其中一项作业，目前被卡住了。无法找出出现此错误的原因。你知道吗

from django.db import models

# Create your models here.
class Pizza(models.Model):
    """List of pizzas"""
    name = models.CharField(max_length=50)

    def __str__(self):
        """Return a string representation of the model"""
        return self.name

class Topping(models.Model):
    """Toppings"""
    pizza = models.ForeignKey(Pizza)
    name = models.CharField(max_length=50)

    def __str__(self):
        """Return a string representation of the model"""
        return self.name

在代码中添加模型顶部时出错。你知道吗

    Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pp_env\lib\site-packages\django\core\management\__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pp_env\lib\site-packages\django\core\management\__init__.py", line 347, in execute
    django.setup()
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pp_env\lib\site-packages\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pp_env\lib\site-packages\django\apps\registry.py", line 112, in populate
    app_config.import_models()
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pp_env\lib\site-packages\django\apps\config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Python36_64\lib\importlib\__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pizzas\models.py", line 12, in <module>
    class Topping(models.Model):
  File "C:\Users\Samuel Tee\Desktop\pizzeria\pizzas\models.py", line 14, in Topping
    pizza = models.ForeignKey(Pizza)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

Answer 1

将on_delete添加到pizzaForeignKey。你知道吗

class Topping(models.Model):
    """Toppings"""
    pizza = models.ForeignKey(Pizza, on_delete=models.CASCADE)

on_delete在Django 2中是必需的-https://docs.djangoproject.com/en/2.0/releases/2.0/#features-removed-in-2-0：

The on_delete argument for ForeignKey and OneToOneField is now required in models and migrations. Consider squashing migrations so that you have fewer of them to update.

Answer 2

问题来自外键，外键定义如下：

pizza = models.ForeignKey('Pizza', on_delete=models.CASCADE)