chore(aci): set DetectorGroup detector to null when it's deleted (#103348)

Author: cathteng

migrations_lockfile.txt

@@ -39,4 +39,4 @@ tempest: 0001_squashed_0002_make_message_type_nullable
 
 uptime: 0048_delete_uptime_status_columns
 
-workflow_engine: 0097_add_unique_constraint_to_datasource
+workflow_engine: 0098_detectorgroup_detector_set_null

src/sentry/workflow_engine/migrations/0098_detectorgroup_detector_set_null.py

@@ -0,0 +1,39 @@
+# Generated by Django 5.2.1 on 2025-11-13 23:59
+
+import django.db.models.deletion
+from django.db import migrations
+
+import sentry.db.models.fields.foreignkey
+from sentry.new_migrations.migrations import CheckedMigration
+
+
+class Migration(CheckedMigration):
+    # This flag is used to mark that a migration shouldn't be automatically run in production.
+    # This should only be used for operations where it's safe to run the migration after your
+    # code has deployed. So this should not be used for most operations that alter the schema
+    # of a table.
+    # Here are some things that make sense to mark as post deployment:
+    # - Large data migrations. Typically we want these to be run manually so that they can be
+    #   monitored and not block the deploy for a long period of time while they run.
+    # - Adding indexes to large tables. Since this can take a long time, we'd generally prefer to
+    #   run this outside deployments so that we don't block them. Note that while adding an index
+    #   is a schema change, it's completely safe to run the operation after the code has deployed.
+    # Once deployed, run these manually via: https://develop.sentry.dev/database-migrations/#migration-deployment
+
+    is_post_deployment = False
+
+    dependencies = [
+        ("workflow_engine", "0097_add_unique_constraint_to_datasource"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="detectorgroup",
+            name="detector",
+            field=sentry.db.models.fields.foreignkey.FlexibleForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                to="workflow_engine.detector",
+            ),
+        ),
+    ]

src/sentry/workflow_engine/models/detector_group.py

@@ -12,7 +12,7 @@ class DetectorGroup(DefaultFieldsModel):
 
     __relocation_scope__ = RelocationScope.Excluded
 
-    detector = FlexibleForeignKey("workflow_engine.Detector", on_delete=models.CASCADE)
+    detector = FlexibleForeignKey("workflow_engine.Detector", null=True, on_delete=models.SET_NULL)
     group = FlexibleForeignKey("sentry.Group", on_delete=models.CASCADE)
 
     class Meta: