Rework schedule escalation chains/policies count as a subquery

engine/apps/api/serializers/schedule_base.py

@@ -76,7 +76,8 @@ class ScheduleBaseSerializer(EagerLoadingMixin, serializers.ModelSerializer):
     def get_number_of_escalation_chains(self, obj):
         # num_escalation_chains param added in queryset via annotate. Check ScheduleView.get_queryset
         # return 0 for just created schedules
-        return getattr(obj, "num_escalation_chains", 0)
+        num = getattr(obj, "num_escalation_chains", 0)
+        return num if num is not None else 0
 
     def validate(self, attrs):
         if "slack_channel_id" in attrs:

engine/apps/api/views/schedule.py

@@ -13,7 +13,7 @@ from rest_framework.permissions import IsAuthenticated
 from rest_framework.views import Response
 from rest_framework.viewsets import ModelViewSet
 
-from apps.alerts.models import EscalationChain
+from apps.alerts.models import EscalationChain, EscalationPolicy
 from apps.api.permissions import MODIFY_ACTIONS, READ_ACTIONS, ActionPermission, AnyRole, IsAdmin, IsAdminOrEditor
 from apps.api.serializers.schedule_base import ScheduleFastSerializer
 from apps.api.serializers.schedule_polymorphic import (
@@ -108,13 +108,16 @@ class ScheduleView(
             slack_team_identity=organization.slack_team_identity,
             slack_id=OuterRef("channel"),
         )
+        escalation_policies = (
+            EscalationPolicy.objects.values("notify_schedule")
+            .order_by("notify_schedule")
+            .annotate(num_escalation_chains=Count("notify_schedule"))
+            .filter(notify_schedule=OuterRef("id"))
+        )
         queryset = queryset.annotate(
             slack_channel_name=Subquery(slack_channels.values("name")[:1]),
             slack_channel_pk=Subquery(slack_channels.values("public_primary_key")[:1]),
-            num_escalation_chains=Count(
-                "escalation_policies__escalation_chain",
-                distinct=True,
-            ),
+            num_escalation_chains=Subquery(escalation_policies.values("num_escalation_chains")[:1]),
         )
         return queryset