Django Model ForeignKey.on_delete=models.CASCADE

首先我们来熟悉一下Django Model中外键的定义方式：

class ForeignKey(to, on_delete, **options)

A many-to-one relationship. Requires two positional arguments: the class to which the model is related and the on_delete option.

对于多对一的关系定义，我们需要传递两个位参，一个是要参照的模型，一个是on_delete选项

To create a recursive relationship – an object that has a many-to-one relationship with itself – use models.ForeignKey(‘self’, on_delete=models.CASCADE).

如果想要创建一个递归的关系参照，比如以自己来创建多对一关系，使用models.ForeignKey('self', on_delete=models.CASCADE)

If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:

from django.db import models

class Car(models.Model):
    manufacturer = models.ForeignKey(
        'Manufacturer',
        on_delete=models.CASCADE,
    )
    # ...

class Manufacturer(models.Model):
    # ...
    pass

关于Model定义时的on_delete=models.CASCADE参数

When the referenced object is deleted, also delete the objects that have references to it (When you remove a blog post for instance, you might want to delete comments as well). SQL equivalent: CASCADE.
当被参照对象被删除的时候，对应的删除与他有参照关系的对象（例如当你删除一篇博客的时候，所有的评论也会被删除），这个定义与SQL中的CASCADE相等（ON DELETE CASCADE）

The other possible values for on_delete are found in django.db.models:

PROTECT

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

SET_NULL

Set the ForeignKey null; this is only possible if null is True.

SET_DEFAULT

Set the ForeignKey to its default value; a default for the ForeignKey must be set.

SET()

Set the ForeignKey to the value passed to SET(), or if a callable is passed in, the result of calling it. In most cases, passing a callable will be necessary to avoid executing queries at the time your models.py is imported:

from django.conf import settings
from django.contrib.auth import get_user_model
from django.db import models

def get_sentinel_user():
    return get_user_model().objects.get_or_create(username='deleted')[0]

class MyModel(models.Model):
    user = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.SET(get_sentinel_user),
    )

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.