Fix 'helpers_required_total' metric

shiftregister/metrics/views.py

@@ -1,6 +1,5 @@
 from django.db import models
 from django.db.models import Count, Case, F, When, Sum
-from django.db.models.functions import Coalesce
 from django.http import HttpResponse
 from shiftregister.app.models import Helper, Shift, ShiftRegistration, Message
 
@@ -42,8 +41,11 @@ def metrics(request):
                         "helpers_required_total",
                         Shift.objects.filter(deleted=False)
                         .annotate(
-                            real_required_helpers=Coalesce(
-                                F("required_helpers"), F("room__required_helpers")
+                            real_required_helpers=Case(
+                                When(
+                                    required_helpers=0, then=F("room__required_helpers")
+                                ),
+                                default=F("required_helpers"),
                             )
                         )
                         .aggregate(sum=Sum("real_required_helpers"))["sum"],