manage.py (python manage.py makemigrations)错误

Question

因此，我是Python和Django的新手，下面是Vitor链接的教程：https://simpleisbetterthancomplex.com/series/2017/09/11/a-complete-beginners-guide-to-django-part-2.html

我将这段代码复制到models.py中

from django.db import models
from django.contrib.auth.models import User


# Create your models here.
class Board(models.Model):
    name = models.CharField(max_length=30, unique=True)
    description = models.CharField(max_length=100)


class Topic(models.Model):
    subject = models.CharField(max_length=255)
    last_updated = models.DateTimeField(auto_now_add=True)
    board = models.ForeignKey(Board, related_name='topics')
    starter = models.ForeignKey(User, related_name='topics')


class Post(models.Model):
    message = models.TextField(max_length=4000)
    topic = models.ForeignKey(Topic, related_name='posts')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(null=True)
    created_by = models.ForeignKey(User, related_name='posts')
    updated_by = models.ForeignKey(User, null=True, related_name='+')

现在，当我在cmd上运行manage.py时，python manage.py makemigrations

我知道这个错误

Traceback (most recent call last):
  File "manage.py", line 21, in <module>
    main()
  File "manage.py", line 17, in main
    execute_from_command_line(sys.argv)
  File "C:\Users\admin\development\myproject\env\lib\site-packages\django\core\management\__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "C:\Users\admin\development\myproject\env\lib\site-packages\django\core\management\__init__.py", line 377, in execute
    django.setup()
  File "C:\Users\admin\development\myproject\env\lib\site-packages\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "C:\Users\admin\development\myproject\env\lib\site-packages\django\apps\registry.py", line 114, in populate
    app_config.import_models()
  File "C:\Users\admin\development\myproject\env\lib\site-packages\django\apps\config.py", line 211, in import_models
    self.models_module = import_module(models_module_name)
  File "C:\Users\admin\development\myproject\env\lib\importlib\__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "C:\Users\admin\development\myproject\myproject\boards\models.py", line 12, in <module>
    class Topic(models.Model):
  File "C:\Users\admin\development\myproject\myproject\boards\models.py", line 15, in Topic
    board = models.ForeignKey(Board, related_name='topics')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

我无法解决这个错误。

Answer 1

topic = models.ForeignKey(Topic, related_name='posts',  on_delete=models.CASCADE)

在删除引用对象时，必须在上添加所有外键要采用的删除行为，请参考https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ForeignKey.on_delete

Answer 2

这是我们创建外键时要传递的标志。它对所有外键列都很有用。有多种选择，但最安全的方法是使用models.PROTECT

解决方案

 models.ForeignKey(Board, on_delete=models.PROTECT)

https://docs.djangoproject.com/en/2.0/ref/models/fields/#django.db.models.ForeignKey.on_delete