Django database migration: delete doesn't work properly

Question

I am using Django 2.2 and psql 10.12 in Ubuntu 18.04. At migration I am trying to remove some rows in database table auth_permission.

from django.db import migrations
from django.core.exceptions import ObjectDoesNotExist
from django.db.models import Q


def remove_rows_at_migrate(apps, schema_editor):
    try:
        permission_model = apps.get_model("auth", "Permission")
        db_alias = schema_editor.connection.alias
        permissions = permission_model.objects.using(db_alias).filter(Q(name='Can do X') |
                                                                  Q(name='Can do Y'))

        for perm in permissions:
            print("deleting: {}".format(perm.name))
            perm.delete()

    except (ObjectDoesNotExist, ValueError):
        pass


class Migration(migrations.Migration):
    dependencies = [
        ('abc', 'xxx'),
    ]

    operations = [
        migrations.RunPython(remove_rows_at_migrate)
    ]

when I migrate, migration is applied and result is [OK] but when I check table auth_persmission "Can do X" and "Can do Y" is still in the table with a new "id".

I can delete rows from same table in manage.py shell_plus without any problems with the following:

>>> permissions = Permission.objects.filter(Q(name='Can do X') |Q(name='Can do Y'))
>>> permissions.delete()

What could cause this behavior?

Answer 1

Because permissions are created using post_migrate signal rather than migrations, see discussions here