Strip - character to make sure field can be validated properly

program/views.py

@@ -541,7 +541,7 @@ class APITimeSlotViewSet(viewsets.ModelViewSet):
         # If someone tries to sort by a field that isn't available on the model
         # we silently ignore that and use the default sort order.
         model_fields = [field.name for field in TimeSlot._meta.get_fields()]
-        if order not in model_fields:
+        if order.lstrip('-') not in model_fields:
             order = default_order