manytomany relation does not exist. It's in a different schema

Question

I use AbstractBaseUser together with CustomPermissionsMixin. CustomPermissionsMixin is kind of the same with django PermissionsMixin the difference is I changed related_name and related_query_name for user_permissions and groups so it won't clashing with django PermissionsMixin related_name

@python_2_unicode_compatible
class CustomPermissionsMixin(models.Model):
    """
    A mixin class that adds the fields and methods necessary to support
    Django's Group and Permission model using the ModelBackend.
    """
    is_superuser = models.BooleanField(
        _('superuser status'),
        default=False,
        help_text=_(
            'Designates that this user has all permissions without '
            'explicitly assigning them.'
        ),
    )
    groups = models.ManyToManyField(
        Group,
        verbose_name=_('groups'),
        blank=True,
        help_text=_(
            'The groups this user belongs to. A user will get all permissions '
            'granted to each of their groups.'
        ),
        related_name="%(app_label)s_%(class)s_related",
        related_query_name="%(app_label)s_%(class)ss",
    )
    user_permissions = models.ManyToManyField(
        Permission,
        verbose_name=_('student user permissions'),
        blank=True,
        help_text=_('Specific permissions for this user.'),
        related_name="%(app_label)s_%(class)s_related",
        related_query_name="%(app_label)s_%(class)ss",
    )

    class Meta:
        abstract = True
    ....

I have the same Student class in two different apps. One is in App1 and another in App2 with slightly different fields. I use postgresql. App1 is in schema public while App2 in schema krt5jdjtrx.(using django tenant schema. created programmatically) Both uses AbstractBaseUser and CustomPermissionsMixin

class Student(AbstractBaseUser, CustomPermissionsMixin):
    ...

I also use DRF DjangoModelPermissions

REST_FRAMEWORK = {
    'DEFAULT_PERMISSION_CLASSES': (
        'rest_framework.permissions.IsAuthenticated',
        'rest_framework.permissions.DjangoModelPermissions',

And custom authentication backend

class CustomBackend(ModelBackend):
    ....

The problem is at _get_user_permissions inside django ModelBackend. Say user_obj is of type app1.Student, user_obj.user_permissions.all().query sometimes use app1_student_user_permissions or app2_student_user_permissions. How come the query uses app2_student_user_permissions while the user_obj is indeed app1 not app2? . It'll create django.db.utils.ProgrammingError: relation does not exist.

def _get_user_permissions(self, user_obj):
    print('inside _get_user_perm !!!!!!!!!!!!!!!!!!!!!!!')
    print(user_obj)
    print(type(user_obj))
    print(user_obj.user_permissions.all().query)
    return user_obj.user_permissions.all()

this is the raw queryset

SELECT "auth_permission"."id", "auth_permission"."name", "auth_permission"."content_type_id", "auth_permission"."codename" FROM "auth_permission" INNER JOIN "app2_student_user_permissions" ON ("auth_permission"."id" = "app2_student_user_permissions"."permission_id") INNER JOIN "django_content_type" ON ("auth_permission"."content_type_id" = "django_content_type"."id") WHERE "app2_student_user_permissions"."student_id" = 1 ORDER BY "django_content_type"."app_label" ASC, "django_content_type"."model" ASC, "auth_permission"."codename" ASC

---

EDIT

App2 student's schema/table will not be created until some point later in the program. Since App2 student has manytomany relation to Permissions, Permissions now has app1 relation and app2 relation. I think it's registered by ManyRelatedManager. (Permissions sees these two relations as public schema)

If I do student1_of_app1.user_permissions.all(), Django will iterate over the relations that Permissions has. Including the non existing App2 table. Thus it'll create django.db.utils.ProgrammingError: relation does not exist.

However, sometimes there is no error because Django gets into app1 relation first, but sometimes Django gets into app2 relation, hence the error.

How can I prevent this from happening?

Answer 1

I found the problem was about django tenant schemas and not django. The migrate_schema --shared actually migrates the whole makemigration files regardless of the app being shared or tenant. Both apps (shared and tenant) are registered in django_content_type table which also registered in auth_permissions. Hence, the relation doesn't exist because tenant tables are not created yet at that point but manytomany relation has been registered for Permissions.