Don't initialise anonymous user on DB where it's not migrated

If the post-migrate signal is fired on a database which doesn't have the user model, skip the logic implemented `get_init_anonymous_user()`. This might be the case when a database router is used to have session somewhere else.

Fixes #650

guardian/management/__init__.py

@@ -1,8 +1,7 @@
-import django
 from django.contrib.auth import get_user_model
 from django.db.models import signals
 from django.utils.module_loading import import_string
-
+from django.db import router
 from guardian.conf import settings as guardian_settings
 
 
@@ -26,6 +25,8 @@ def create_anonymous_user(sender, **kwargs):
     Creates anonymous User instance with id and username from settings.
     """
     User = get_user_model()
+    if not router.allow_migrate_model(kwargs['using'], User):
+        return
     try:
         lookup = {User.USERNAME_FIELD: guardian_settings.ANONYMOUS_USER_NAME}
         User.objects.using(kwargs['using']).get(**lookup)

guardian/testapp/tests/test_management.py

@@ -1,4 +1,8 @@
+from copy import deepcopy
+
+from django.conf import settings
 from django.contrib.auth import get_user_model
+from django.contrib.sessions.models import Session
 from django.test import TestCase, override_settings
 
 import mock
@@ -7,6 +11,26 @@
 
 
 mocked_get_init_anon = mock.Mock()
+multi_db_dict = {
+    'default': deepcopy(settings.DATABASES['default']),
+    'session': deepcopy(settings.DATABASES['default']),
+}
+
+
+class SessionRouter:
+    @staticmethod
+    def db_for_write(model, **kwargs):
+        if model == Session:
+            return 'session'
+        else:
+            return None
+
+    @staticmethod
+    def allow_migrate(db, app_label, **kwargs):
+        if db == 'session':
+            return app_label == 'sessions'
+        else:
+            return None
 
 
 class TestGetAnonymousUser(TestCase):
@@ -45,3 +69,16 @@ def test_get_anonymous_user(self):
         anon = get_anonymous_user()
         self.assertFalse(anon.has_usable_password())
         self.assertEqual(anon.get_username(), "AnonymousUser")
+
+    @mock.patch('guardian.management.guardian_settings')
+    @override_settings(
+        DATABASE_ROUTERS=[SessionRouter()],
+        DATABASES=multi_db_dict
+    )
+    def test_non_migrated_db(self, guardian_settings):
+        mocked_get_init_anon.reset_mock()
+        guardian_settings.GET_INIT_ANONYMOUS_USER = 'guardian.testapp.tests.test_management.mocked_get_init_anon'
+
+        create_anonymous_user('sender', using='session')
+
+        mocked_get_init_anon.assert_not_called()