这是你的问题的答案，说:为什么我们使用on_delete?

当一个被ForeignKey引用的对象被删除时，Django默认会模仿SQL约束ON DELETE CASCADE的行为，同时也会删除包含ForeignKey的对象。可以通过指定on_delete参数来重写此行为。例如，如果你有一个可为空的ForeignKey，并且你想在删除引用对象时将它设置为空:

user = models.ForeignKey(User, blank=True, null=True, on_delete=models.SET_NULL)

on_delete的可能值可以在django.db.models中找到:

CASCADE:级联删除;默认值。

PROTECT:通过引发ProtectedError (django.db.IntegrityError的子类)来防止被引用对象的删除。

SET_NULL:设置ForeignKey为空;这只有在null为True时才有可能。

SET_DEFAULT:设置ForeignKey为默认值;必须设置一个默认的ForeignKey。

CASCADE也会删除与之相连的对应字段。

简单地说，on_delete是一条指令，用于指定在删除外部对象时将对对象进行哪些修改:

CASCADE:当外部对象被删除时，将删除子对象

SET_NULL:将子对象外键设置为null

SET_DEFAULT:在创建模型时将子对象设置为给定的默认数据

RESTRICT:在某些条件下引发RestrictedError。

PROTECT:防止外部对象被删除，只要有子对象从它继承

额外的链接:

https://docs.djangoproject.com/en/4.0/ref/models/fields/#foreignkey

假设您有两个模型，一个名为Person，另一个名为Companies。

根据定义，一个人可以创建多家公司。

考虑到一个公司可以有且只能有一个人，我们希望当一个人被删除时，与他相关的所有公司也都被删除。

我们首先创建一个Person模型，像这样

class Person(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.id+self.name

然后，Companies模型看起来像这样

class Companies(models.Model):
    title = models.CharField(max_length=20)
    description=models.CharField(max_length=10)
    person= models.ForeignKey(Person,related_name='persons',on_delete=models.CASCADE)

注意on_delete=models的用法。模型公司中的CASCADE。也就是说，当拥有公司的人(类person的实例)被删除时，删除所有公司。

如前所述，CASCADE将删除具有外键的记录，并引用已删除的另一个对象。例如，如果你有一个房地产网站，有一个引用城市的属性

class City(models.Model):
    # define model fields for a city

class Property(models.Model):
    city = models.ForeignKey(City, on_delete = models.CASCADE)
    # define model fields for a property

现在当城市从数据库中删除时，所有相关的属性(例如。位于该城市的房地产)也将从数据库中删除

现在我还想提一下其他选项的优点，比如SET_NULL或SET_DEFAULT甚至DO_NOTHING。基本上，从管理的角度来看，您希望“删除”这些记录。但你不希望它们消失。原因有很多。有人可能不小心删除了它，或者是为了审计和监控。以及简单的报道。因此，这可能是一种将房产与城市“断开”的方法。同样，这取决于您的应用程序是如何编写的。

For example, some applications have a field "deleted" which is 0 or 1. And all their searches and list views etc, anything that can appear in reports or anywhere the user can access it from the front end, exclude anything that is deleted == 1. However, if you create a custom report or a custom query to pull down a list of records that were deleted and even more so to see when it was last modified (another field) and by whom (i.e. who deleted it and when)..that is very advantageous from the executive standpoint.

不要忘记，对于这些记录，您可以恢复意外删除，只需删除= 0即可。

我的观点是，如果有一个功能，它背后总是有一个原因。这并不总是一个好理由。但是有一个原因。而且通常都是很好的。

供您参考，模型中的on_delete参数与它听起来的相反。你把on_delete放在一个模型的外键(FK)上，告诉Django如果你在记录上指向的FK条目被删除了该怎么办。我们商店使用最多的选项是PROTECT、CASCADE和SET_NULL。以下是我总结出的基本规则:

Use PROTECT when your FK is pointing to a look-up table that really shouldn't be changing and that certainly should not cause your table to change. If anyone tries to delete an entry on that look-up table, PROTECT prevents them from deleting it if it is tied to any records. It also prevents Django from deleting your record just because it deleted an entry on a look-up table. This last part is critical. If someone were to delete the gender "Female" from my Gender table, I CERTAINLY would NOT want that to instantly delete any and all people I had in my Person table who had that gender. Use CASCADE when your FK is pointing to a "parent" record. So, if a Person can have many PersonEthnicity entries (he/she can be American Indian, Black, and White), and that Person is deleted, I really would want any "child" PersonEthnicity entries to be deleted. They are irrelevant without the Person. Use SET_NULL when you do want people to be allowed to delete an entry on a look-up table, but you still want to preserve your record. For example, if a Person can have a HighSchool, but it doesn't really matter to me if that high-school goes away on my look-up table, I would say on_delete=SET_NULL. This would leave my Person record out there; it just would just set the high-school FK on my Person to null. Obviously, you will have to allow null=True on that FK.

下面是一个模型的例子，它可以做到这三件事:

class PurchPurchaseAccount(models.Model):
    id = models.AutoField(primary_key=True)
    purchase = models.ForeignKey(PurchPurchase, null=True, db_column='purchase', blank=True, on_delete=models.CASCADE) # If "parent" rec gone, delete "child" rec!!!
    paid_from_acct = models.ForeignKey(PurchPaidFromAcct, null=True, db_column='paid_from_acct', blank=True, on_delete=models.PROTECT) # Disallow lookup deletion & do not delete this rec.
    _updated = models.DateTimeField()
    _updatedby = models.ForeignKey(Person, null=True, db_column='_updatedby', blank=True, related_name='acctupdated_by', on_delete=models.SET_NULL) # Person records shouldn't be deleted, but if they are, preserve this PurchPurchaseAccount entry, and just set this person to null.

    def __unicode__(self):
        return str(self.paid_from_acct.display)
    class Meta:
        db_table = u'purch_purchase_account'

作为最后的花边新闻，您知道如果您没有指定on_delete(或没有指定)，默认行为是CASCADE吗?这意味着如果有人删除了您的gender表中的一个性别条目，那么具有该性别的任何Person记录也会被删除!

我会说，“如果有疑问，设置on_delete=models.PROTECT。”然后测试你的应用程序。您将很快发现哪些fk应该标记为其他值，而不会危及任何数据。

此外，值得注意的是，on_delete=CASCADE实际上没有添加到任何迁移中，如果这是您选择的行为的话。我想这是因为它是默认的，所以把on_delete=CASCADE等同于什么都不放。