Ensure add_stator_indexes only run for direct descendants of StatorModel

stator/models.py

@@ -44,8 +44,11 @@ def add_stator_indexes(sender, **kwargs):
  Inject Indexes used by StatorModel in to any subclasses. This sidesteps the
  current Django inability to inherit indexes when the Model subclass defines
  its own indexes.
+
+ We should only add this for models that direct descendants of
+ StatorModel, otherwise we will see system check errors.
  """
- if issubclass(sender, StatorModel):
+ if sender.__base__ is StatorModel:
  indexes = [
  models.Index(
  fields=["state", "state_next_attempt", "state_locked_until"],