Django migration fails with "__fake__.DoesNotExist: Permission matching query does not exist."
Question: In a Django 1.8 project, I have a migration that worked fine, [when it had the
following
code](https://github.com/geometalab/osmaxx/blob/378ddc5043f1fd80727067de19316f30d1f725b5/osmaxx-
py/osmaxx/contrib/auth/migrations/0002_add_default_usergroup_osmaxx.py):

    
    
    # -*- 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: 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.

