为什么 Django on_delete 规则不起作用？

我有两个模型通过外键连接，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，尝试删除一些相册：

# -*- 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".

But the on_delete规则就在那里。删除规则的全部目的不就是为了避免这样的错误吗？或者我错过了什么？最初我正在尝试delete在查询集上，我认为on_delete不支持规则，我必须循环查询集并在每个实例上调用删除。但显然这还不够。重点是什么on_delete如果这也不起作用？有什么我可以做的吗？谢谢。

UPDATE: 在 shell 中，它可以工作。我只在迁移过程中遇到错误。

你需要允许album确实为空：

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