Question

我有两个通过ForeignKey进行连接的模型，如下所示：

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)
    # ...

我正在编写数据迁移，尝试删除某些相册：

# -*- 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，我遇到此错误：

  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条规则。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，我在查询集上尝试使用delete，但我认为不支持on_delete规则，因此我必须遍历查询集并在每个实例上调用delete。但是显然这还不够。如果on_delete仍然不起作用，那又有什么意义呢？有什么我可以做的吗？谢谢。

更新：在外壳程序中有效。我只会在迁移中遇到错误。

Answer 1

您实际上需要允许album为空：

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

为什么Django on_delete规则不起作用？

1 个答案: