Django error relation "auth_user" does not exist using multiple database - django

Question

I'm working on Django 1.9 and python 3.3 project using multiple databases (different schema in a same postgresql database). When I try to migrate the project for the first time, I'm getting this error
Running migrations:
Rendering model states... DONE
Applying contenttypes.0001_initial... OK
Applying auth.0001_initial... OK
Applying MyApp.0001_initial...Traceback (most recent call last):
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: ERROR: relation "auth_user" does not exist
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "manage.py", line 10, in
execute_from_command_line(sys.argv)
File "/usr/lib/python3.4/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
utility.execute()
File "/usr/lib/python3.4/site-packages/django/core/management/__init__.py", line 342, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/usr/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
self.execute(*args, **cmd_options)
File "/usr/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
output = self.handle(*args, **options)
File "/usr/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 200, in handle
executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 92, in migrate
self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 198, in apply_migration
state = migration.apply(state, schema_editor)
File "/usr/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 90, in __exit__
self.execute(sql)
File "/usr/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 110, in execute
cursor.execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 79, in execute
return super(CursorDebugWrapper, self).execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/utils.py", line 95, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/usr/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
raise value.with_traceback(tb)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: ERROR: relation "auth_user" does not exist
This error seems to appeard in other projects when the auth_user table isn't migrated. In my case beginning my migration with manage.py migrate auth before migrating the app who need it doesn't solve the problem.
I suspect that the problem come from the use of different database in Django. My auth_user table is stored in the default database and the content of the models.py is routed to an other database.
Is the migration process looking for the auth_user table in the same database than my app database? Is it something absolutely different?

Answer 1

Answer to the problem
It is in fact a cross database reference problem. Django can't create cross database foreign keys.
From the Django 1.8 documentation (and there isn't any solution in the next versions (current version is 1.10)) :
Cross-database relations
Django doesn’t currently provide any support for foreign key or
many-to-many relationships spanning multiple databases. If you have
used a router to partition models to different databases, any foreign
key and many-to-many relationships defined by those models must be
internal to a single database.
This is because of referential integrity. In order to maintain a
relationship between two objects, Django needs to know that the
primary key of the related object is valid. If the primary key is
stored on a separate database, it’s not possible to easily evaluate
the validity of a primary key.
If you’re using Postgres, Oracle, or MySQL with InnoDB, this is
enforced at the database integrity level – database level key
constraints prevent the creation of relations that can’t be validated.
However, if you’re using SQLite or MySQL with MyISAM tables, there is
no enforced referential integrity; as a result, you may be able to
‘fake’ cross database foreign keys. However, this configuration is not
officially supported by Django
How to workaround and keep separate databases
In my case, because router are working, there is a little hack to simplify the cross database objects.
class CrossDBUser(models.Model):
user = models.IntegerField()
def get_user(self):
return User.objects.get(id=self.user)
def set_user(self, user):
self.user = user.id
class MyClassWithCrossDB(CrossDBUser):
field1 = models.CharField(max_length=200, blank=False)
field2 = models.IntegerField(default=0)
With this I can use the method set_user and get_user to work with the user stored in my object MyClassWithCrossDB.
Of course it's not perfect because it doesn't allow automated actions like on_delete=models.CASCADE and I'm obliged to work with methods instead of the instance variable. But it is a solution to workaround.