Programmatically creating a group : Can't access permissions from migration - django

Question

After seeing this post, I tried to create my own group at project setup with this migration :
from django.db import migrations
from django.contrib.auth.models import Group, Permission
def create_group(apps, schema_editor):
group, created = Group.objects.get_or_create(name='thing_managers')
if created:
add_thing = Permission.objects.get(codename='add_thing')
group.permissions.add(add_thing)
group.save()
class Migration(migrations.Migration):
dependencies = [
('main', '0002_auto_20160720_1809'),
]
operations = [
migrations.RunPython(create_group),
]
But I got the following error :
django.contrib.auth.models.DoesNotExist: Permission matching query does not exist.
Here is my model :
class Thing(models.Model):
pass
Why can't I do that? How could I solve this?
I use django 1.9.

Answer 1

Permissions are created in a post_migrate signal. They don't exist the first time migrations are run after a new model is added. It is probably easiest to run the post_migrate signal handler manually:
from django.contrib.auth.management import create_permissions
def create_group(apps, schema_editor):
for app_config in apps.get_app_configs():
create_permissions(app_config, apps=apps, verbosity=0)
group, created = Group.objects.get_or_create(name='thing_managers')
if created:
add_thing = Permission.objects.get(codename='add_thing')
group.permissions.add(add_thing)
group.save()
create_permissions checks for existing permissions, so this won't create any duplicates.

Answer 2

From this Django ticket, here's what worked for me in Django 3.0.4 and apparently will work in >=1.9:
from django.core.management.sql import emit_post_migrate_signal
def create_group(apps, schema_editor):
# Ensure permissions and content types have been created.
db_alias = schema_editor.connection.alias
emit_post_migrate_signal(2, False, db_alias)
# Now the content types and permissions should exist
Permission = apps.get_model('auth', 'Permission')
...

Answer 3

One solution is call the update_permissions command before try to append a permission
from django.core.management import call_command
def update_permissions(schema, group):
call_command('update_permissions')
operations = [
migrations.RunPython(update_permissions, reverse_code=migrations.RunPython.noop),
migrations.RunPython(create_group),
]
And as was commented don't import Group and Permission models use:
Group = apps.get_model("auth","Group")
Permission = apps.get_model("auth","Permission")