fix(deletions): Avoid DoesNotExist crash when FK target is already deleted

DetectorDeletionTask and WorkflowDeletionTask both accessed FK descriptors
(instance.workflow_condition_group, instance.when_condition_group) to check
whether to include a related DataConditionGroup in child relations. When the
referenced row has already been deleted, Django raises DoesNotExist.

Because _run_deletion swallows all exceptions in production without re-raising,
this caused the deletion task to silently fail and enter an infinite retry loop
via reattempt_deletions. Orgs with detectors or workflows in this state can
stay stuck in DELETION_IN_PROGRESS indefinitely.

Fix by reading the raw FK id field instead of going through the descriptor,
which avoids the database lookup and the potential DoesNotExist entirely.

Fixes SENTRY-5M8C
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: swartzrock

src/sentry/deletions/defaults/detector.py

@@ -26,9 +26,9 @@ def get_child_relations(self, instance: Detector) -> list[BaseRelation]:
             ):
                 model_relations.append(ModelRelation(DataSource, {"detector": instance.id}))
 
-        if instance.workflow_condition_group:
+        if instance.workflow_condition_group_id:
             model_relations.append(
-                ModelRelation(DataConditionGroup, {"id": instance.workflow_condition_group.id})
+                ModelRelation(DataConditionGroup, {"id": instance.workflow_condition_group_id})
             )
 
         return model_relations

src/sentry/deletions/defaults/workflow.py

@@ -21,9 +21,9 @@ def get_child_relations(self, instance: Workflow) -> list[BaseRelation]:
 
             model_relations.append(ModelRelation(DataConditionGroup, {"id__in": action_filter_ids}))
 
-        if instance.when_condition_group:
+        if instance.when_condition_group_id:
             model_relations.append(
-                ModelRelation(DataConditionGroup, {"id": instance.when_condition_group.id})
+                ModelRelation(DataConditionGroup, {"id": instance.when_condition_group_id})
             )
 
         return model_relations

tests/sentry/deletions/test_detector.py

@@ -176,6 +176,20 @@ def test_delete_uptime_detector_succeeds_when_remove_seat_fails(
         # Verify the error path in DetectorDeletionTask was actually exercised.
         mock_remove_seat_subscriptions.assert_called_once()
 
+    def test_dangling_workflow_condition_group(self) -> None:
+        """Deletion succeeds when workflow_condition_group_id references a deleted DataConditionGroup."""
+        # Simulate a dangling FK — points to a non-existent DataConditionGroup row
+        Detector.objects_for_deletion.filter(id=self.detector.id).update(
+            workflow_condition_group_id=999999
+        )
+
+        self.ScheduledDeletion.schedule(instance=self.detector, days=0)
+
+        with self.tasks():
+            run_scheduled_deletions()
+
+        assert not Detector.objects_for_deletion.filter(id=self.detector.id).exists()
+
     def test_delete_uptime_subscription_without_detector(self) -> None:
         """UptimeSubscription deletion proceeds when the detector no longer exists."""
         detector = self.create_uptime_detector()

tests/sentry/deletions/test_workflow.py

@@ -59,6 +59,22 @@ def setUp(self) -> None:
         self.workflow.status = ObjectStatus.PENDING_DELETION
         self.workflow.save()
 
+    def test_dangling_when_condition_group(self) -> None:
+        """Deletion succeeds when when_condition_group_id references a deleted DataConditionGroup."""
+        from sentry.workflow_engine.models import Workflow
+
+        # Simulate a dangling FK — points to a non-existent DataConditionGroup row
+        Workflow.objects_for_deletion.filter(id=self.workflow.id).update(
+            when_condition_group_id=999999
+        )
+
+        self.ScheduledDeletion.schedule(instance=self.workflow, days=0)
+
+        with self.tasks():
+            run_scheduled_deletions()
+
+        assert not Workflow.objects_for_deletion.filter(id=self.workflow.id).exists()
+
     @pytest.mark.parametrize(
         "instance_attr",
         [