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。

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。

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。

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

删除父对象时删除数据库中的所有子字段，然后使用on_delete，如下所示:

class user(models.Model):
    commodities = models.ForeignKey(commodity, on_delete=models.CASCADE)

重新定位你对“CASCADE”功能的思维模式，将FK添加到一个已经存在的CASCADE(即瀑布)中。这个瀑布的源是一个主键(PK)。删除向下流动。

因此，如果您将一个FK的on_delete定义为“CASCADE”，那么您将把这个FK的记录添加到一个源于PK的级联删除中。FK的记录可能参与这个级联，也可能不参与(“SET_NULL”)。事实上，具有FK的记录甚至可能阻止删除流!用“PROTECT”建造一座大坝。