为什么Django on_delete规则不起作用？ [英] Why is Django on_delete rule not working?

问题描述

我有两个通过ForeignKey进行连接的模型，如下所示：

I have two models connected via a ForeignKey sort of like this:

class Album(models.Model):
    name = models.CharField(max_length=128)
    # ...

class Track(models.Model):
    name = models.CharField(max_length=128)
    album = models.ForeignKey(Album, related_name='tracks', null=True, on_delete=models.SET_NULL)
    # ...

我正在写数据迁移，尝试删除一些相册：

I'm writing a data migration where I try to delete some Albums:

# -*- coding: utf-8 -*-
# Generated by Django 1.9.13 on 2018-12-12 14:05
from __future__ import unicode_literals

from django.db import migrations


def forwards_func(apps, schema_editor):
    Album = apps.get_model("myapp", "Album")
    db_alias = schema_editor.connection.alias

    for album in Album.objects.using(db_alias).filter(foo='bar'):
        album.delete()

def reverse_func(apps, schema_editor):
    pass

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0049_blabla'),
    ]

    operations = [
        migrations.RunPython(forwards_func, reverse_func),
    ]

但是，我收到此错误：

  File "/usr/local/lib/python2.7/site-packages/django/db/backends/base/base.py", line 211, in _commit
    return self.connection.commit()
IntegrityError: update or delete on table "Album" violates foreign key constraint "f2274d6f2be82bbff458f3e5487b1864" on table "Track"
DETAIL:  Key (id)=(1) is still referenced from table "Track".

但是有 on_delete 规则。删除规则不是要避免这样的错误吗？还是我错过了什么？最初，我尝试在查询集上使用 delete ，但我认为不支持 on_delete 规则，因此我不得不遍历queryset并在每个实例上调用delete。但是显然这还不够。 on_delete 的意思是什么，即使这样也不起作用？有什么我可以做的吗？谢谢。

But the on_delete rule is there. Isn't the whole point of the delete rule to avoid errors like this? Or am I missing something? Initially I was trying a delete on the queryset, and I thought the on_delete rule wasn't supported and I had to loop through the queryset and call a delete on each instance. But apparently not even this is enough. What is the point of on_delete if even this doesn't work? Is there anything I can do? Thanks.

更新：在外壳程序中有效。我只会在迁移中遇到错误。

UPDATE: In the shell, it works. I only get the error in the migration.

推荐答案

您需要允许相册实际上为null：

You need to allow album to be null indeed:

album = models.ForeignKey(Album, on_delete=models.SET_NULL, null=True)

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