Django migration fails with “__fake__.DoesNotExist

Question

In a Django 1.8 project, I have a migration that worked fine, when it had the following code:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations
from django.conf import settings


def update_site_forward(apps, schema_editor):
    """Add group osmaxx."""
    Group = apps.get_model("auth", "Group")
    Group.objects.create(name=settings.OSMAXX_FRONTEND_USER_GROUP)


def update_site_backward(apps, schema_editor):
    """Revert add group osmaxx."""
    Group = apps.get_model("auth", "Group")
    Group.objects.get(name=settings.OSMAXX_FRONTEND_USER_GROUP).delete()


class Migration(migrations.Migration):

    dependencies = [
        ('auth', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(update_site_forward, update_site_backward),
    ]

This group is created in a migration, because it shall be available in all installations of the web app. To make it more useful, I wanted to also give it a default permission, so I changed update_site_forward to:

def update_site_forward(apps, schema_editor):
    """Add group osmaxx."""
    Group = apps.get_model("auth", "Group")
    Permission = apps.get_model("auth", "Permission")
    ContentType = apps.get_model("contenttypes", "ContentType")
    ExtractionOrder = apps.get_model("excerptexport", "ExtractionOrder")
    group = Group.objects.create(name=settings.OSMAXX_FRONTEND_USER_GROUP)
    content_type = ContentType.objects.get_for_model(ExtractionOrder)
    permission = Permission.objects.get(codename='add_extractionorder',
                                        content_type=content_type) # line 16
    group.permissions.add(permission)

and Migration.dependencies to:

    dependencies = [
        ('contenttypes', '0002_remove_content_type_name'),
        ('excerptexport', '0001_initial'),
        ('auth', '0001_initial'),
    ]

While applying the migration (after first reverting it) (python3 manage.py migrate auth 0001 && python3 managy.py migrate) worked, migrating a newly created PostgreSQL database with this and all other migrations (python3 manage.py migrate) fails:

Operations to perform:
  Synchronize unmigrated apps: debug_toolbar, django_extensions, messages, humanize, social_auth, kombu_transport_django, staticfiles
  Apply all migrations: excerptexport, admin, sites, contenttypes, sessions, default, stored_messages, auth
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying auth.0002_add_default_usergroup_osmaxx...Traceback (most recent call last):
  File "manage.py", line 17, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/base.py", line 393, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/base.py", line 444, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.4/dist-packages/django/core/management/commands/migrate.py", line 221, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/executor.py", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/executor.py", line 148, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/migration.py", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/lib/python3.4/dist-packages/django/db/migrations/operations/special.py", line 183, in database_forwards
    self.code(from_state.apps, schema_editor)
  File "/home/osmaxx/source/osmaxx/contrib/auth/migrations/0002_add_default_usergroup_osmaxx.py", line 16, in update_site_forward
    permission = Permission.objects.get(codename='add_extractionorder', content_type=content_type)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/query.py", line 334, in get
    self.model._meta.object_name
__fake__.DoesNotExist: Permission matching query does not exist.

What am I doing wrong?

Answer 1

The default permissions are created in a post_migrate signal handler, after the migrations have run. This won't be a problem if your updated code runs as part of the second manage.py migrate run, but it is a problem in the test suite and any new deployment.

The easy fix is to change this line:

permission = Permission.objects.get(codename='add_extractionorder',
                                    content_type=content_type) # line 16

to this:

permission, created = Permission.objects.get_or_create(codename='add_extractionorder',
                                              content_type=content_type)

The signal handler that creates the default permissions will never create a duplicate permission, so it is safe to create it if it doesn't exist already.