Handle ValueError raised in get_privileges

Django 4 raises a ValueError when attempting to read a fk relationship
on an unsaved model, rather than return an empty list.

django_prbac/models.py

@@ -144,8 +144,12 @@ def get_privileges(self, assignment):
                 return self._granted_privileges
             except AttributeError:
                 pass
-        return [membership.instantiated_to_role(assignment)
-                for membership in self.memberships_granted.all()]
+        try:
+            return [membership.instantiated_to_role(assignment)
+                    for membership in self.memberships_granted.all()]
+        except ValueError:
+            # Django 4 raises ValueError if fk relationship is accessed prior to save
+            return []
 
     def instantiate(self, assignment):
         """

django_prbac/tests/test_models.py

@@ -71,16 +71,10 @@ def test_has_permission_immediate_params(self):
         self.assertFalse(subrole.instantiate({}).has_privilege(superrole1.instantiate(dict(one='baz'))))
 
     def test_unsaved_role_does_not_have_permission(self):
-        import django
         role1 = Role()
         role2 = arbitrary.role()
-        if django.VERSION >= (4, 0):
-            with self.assertRaises(ValueError):
-                self.assertFalse(role1.has_privilege(role2))
-                self.assertFalse(role2.has_privilege(role1))
-        else:
-            self.assertFalse(role1.has_privilege(role2))
-            self.assertFalse(role2.has_privilege(role1))
+        self.assertFalse(role1.has_privilege(role2))
+        self.assertFalse(role2.has_privilege(role1))
 
 
 class TestGrant(TestCase):