Getting TypeError: __init__() missing 1 required positional argument: 'on_delete' when trying to add parent table after child table with entries

问题

I have two classes in my sqlite database, a parent table named Categorie and the child table called Article. I created first the child table class and addes entries. So first I had this:

class Article(models.Model):
    titre=models.CharField(max_length=100)
    auteur=models.CharField(max_length=42)
    contenu=models.TextField(null=True)
    date=models.DateTimeField(
        auto_now_add=True,
        auto_now=False,
        verbose_name="Date de parution"
    )

    def __str__(self):
        return self.titre

And after I have added parent table, and now my models.py looks like this:

from django.db import models

# Create your models here.
class Categorie(models.Model):
    nom = models.CharField(max_length=30)

    def __str__(self):
        return self.nom


class Article(models.Model):
    titre=models.CharField(max_length=100)
    auteur=models.CharField(max_length=42)
    contenu=models.TextField(null=True)
    date=models.DateTimeField(
        auto_now_add=True,
        auto_now=False,
        verbose_name="Date de parution"
    )
    categorie = models.ForeignKey('Categorie')

    def __str__(self):
        return self.titre

So when I run python manage.py makemigrations <my_app_name>, I get this error:

Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Users\lislis\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django-2.0-py3.5.egg\django\core\management\__init__.py", line 354, in execute_from_command_line
    utility.execute()
  File "C:\Users\lislis\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django-2.0-py3.5.egg\django\core\management\__init__.py", line 330, in execute
    django.setup()
  File "C:\Users\lislis\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django-2.0-py3.5.egg\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "C:\Users\lislis\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django-2.0-py3.5.egg\django\apps\registry.py", line 112, in populate
    app_config.import_models()
  File "C:\Users\lislis\AppData\Local\Programs\Python\Python35-32\lib\site-packages\django-2.0-py3.5.egg\django\apps\config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "C:\Users\lislis\AppData\Local\Programs\Python\Python35-32\lib\importlib\__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "C:\Users\lislis\Django\mon_site\blog\models.py", line 6, in <module>
    class Article(models.Model):
  File "C:\Users\lislis\Django\mon_site\blog\models.py", line 16, in Article
    categorie = models.ForeignKey('Categorie')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

I've seen some similar issues in stackoverflow, but it seems to not be the same problem: __init__() missing 1 required positional argument: 'quantity'

回答1:

You can change the property categorie of the class Article like this:

categorie = models.ForeignKey(
    'Categorie',
    on_delete=models.CASCADE,
)

and the error should disappear.

Eventually you might need another option for on_delete, check the documentation for more details:

https://docs.djangoproject.com/en/1.11/ref/models/fields/#django.db.models.ForeignKey

EDIT:

As you stated in your comment, that you don't have any special requirements for on_delete, you could use the option DO_NOTHING:

# ...
on_delete=models.DO_NOTHING,
# ...

回答2:

Since Django 2.x, on_delete is required.

Django Documentation

Deprecated since version 1.9: on_delete will become a required argument in Django 2.0. In older versions it defaults to CASCADE.

回答3:

From Django 2.0 on_delete is required:

user = models.OneToOneField(User, on_delete=models.CASCADE)

It will delete the child table data if the User is deleted. For more details check the Django documentation.

回答4:

If you are using foreignkey then you have to use "on_delete=models.CASCADE" as it will eliminate the complexity developed after deleting the original element from the parent table. As simple as that.

categorie = models.ForeignKey('Categorie', on_delete=models.CASCADE)

回答5:

Since Django 2.0 the ForeignKey field requires two positional arguments:

the model to map to
the on_delete argument

categorie = models.ForeignKey('Categorie', on_delete=models.PROTECT)

Here are some methods can used in on_delete

CASCADE

Cascade deletes. Django emulates the behavior of the SQL constraint ON DELETE CASCADE and also deletes the object containing the ForeignKey

PROTECT

Prevent deletion of the referenced object by raising ProtectedError, a subclass of django.db.IntegrityError.

DO_NOTHING

Take no action. If your database backend enforces referential integrity, this will cause an IntegrityError unless you manually add an SQL ON DELETE constraint to the database field.

you can find more about on_delete by reading the documentation.