13813 fix virtual chassis member count (#13823)

* 13813 fix virtual chassis member count

* 13813 add test

netbox/utilities/counters.py

@@ -52,12 +52,13 @@ def post_save_receiver(sender, instance, created, **kwargs):
     for field_name, counter_name in get_counters_for_model(sender):
         parent_model = sender._meta.get_field(field_name).related_model
         new_pk = getattr(instance, field_name, None)
-        old_pk = instance.tracker.get(field_name) if field_name in instance.tracker else None
+        has_old_field = field_name in instance.tracker
+        old_pk = instance.tracker.get(field_name) if has_old_field else None
 
         # Update the counters on the old and/or new parents as needed
         if old_pk is not None:
             update_counter(parent_model, old_pk, counter_name, -1)
-        if new_pk is not None and (old_pk or created):
+        if new_pk is not None and (has_old_field or created):
             update_counter(parent_model, new_pk, counter_name, 1)
 
 

netbox/utilities/tests/test_counters.py

@@ -36,10 +36,18 @@ class CountersTest(TestCase):
         self.assertEqual(device1.interface_count, 3)
         self.assertEqual(device2.interface_count, 3)
 
+        # test saving an existing object - counter should not change
         interface1.save()
         device1.refresh_from_db()
         self.assertEqual(device1.interface_count, 3)
 
+        # test save where tracked object FK back pointer is None
+        vc = VirtualChassis.objects.create(name='Virtual Chassis 1')
+        device1.virtual_chassis = vc
+        device1.save()
+        vc.refresh_from_db()
+        self.assertEqual(vc.member_count, 1)
+
     def test_interface_count_deletion(self):
         """
         When a tracked object (Interface) is deleted the tracking counter should be updated.