manage.py (python manage.py makemigrations) 错误

Question

So i am new to Python and Django and following a tutorial by Mr. Vitor Freitas link: https://simpleisbetterthancomplex.com/series/2017/09/11/a-complete-beginners-guide-to-django-part-2.html

I copied this code into models.py

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='+')

Now when i run manage.py on cmd python manage.py makemigrations

i get this error

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'

I am not able to resolve the error.

Answer 1

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

当引用的对象被删除时，您必须在所有外键上添加删除行为，请参阅https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ForeignKey .on_delete

Answer 2

Well, it's a flag to be passed when we create foreign key. It's useful for all the foreign key columns. There are multiple options but the most safest way to do this is using models.PROTECT

Solution

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

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