fix(monitors): Allow editing cron monitor detectors with existing slug (#103846)

Fixed [NEW-648: Cron detectors can't be edited due to
error](https://linear.app/getsentry/issue/NEW-648/cron-detectors-cant-be-edited-due-to-error)

When updating a cron monitor detector with dataSources in the request
body,
the slug validation was incorrectly rejecting the existing slug as
"already
in use". This occurred because the MonitorDataSourceValidator nested in
a
ListField didn't have its instance property set during validation.

Created MonitorDataSourceListField to bind the Monitor instance to child
validators before validation runs, allowing the slug check to recognize
updates vs creates.

Author: evanpurkhiser

src/sentry/monitors/validators.py

@@ -686,6 +686,28 @@ def update(self, instance: Monitor, validated_data: dict[str, Any]) -> Monitor:
             return monitor_validator.update(instance, monitor_validator.validated_data)
 
 
+class MonitorDataSourceListField(serializers.ListField):
+    """
+    Custom ListField that properly binds the Monitor instance to child validators.
+
+    When updating a detector, we need to ensure the MonitorDataSourceValidator
+    knows about the existing Monitor so slug validation works correctly.
+    """
+
+    def to_internal_value(self, data):
+        # If we're updating (parent has instance), bind the Monitor instance to child validator
+        if hasattr(self.parent, "instance") and self.parent.instance:
+            detector = self.parent.instance
+            monitor = get_cron_monitor(detector)
+
+            # Bind the monitor instance so slug validation recognizes this as an update
+            # Type ignore: self.child is typed as Field but is actually MonitorDataSourceValidator
+            self.child.instance = monitor  # type: ignore[attr-defined]
+            self.child.partial = self.parent.partial  # type: ignore[attr-defined]
+
+        return super().to_internal_value(data)
+
+
 class MonitorIncidentDetectorValidator(BaseDetectorTypeValidator):
     """
     Validator for monitor incident detection configuration.
@@ -694,7 +716,7 @@ class MonitorIncidentDetectorValidator(BaseDetectorTypeValidator):
     data_source field (MonitorDataSourceValidator).
     """
 
-    data_sources = serializers.ListField(child=MonitorDataSourceValidator(), required=False)
+    data_sources = MonitorDataSourceListField(child=MonitorDataSourceValidator(), required=False)
 
     def validate_enabled(self, value: bool) -> bool:
         """

tests/sentry/monitors/endpoints/test_organization_detector_details.py

@@ -138,6 +138,42 @@ def test_update_monitor_incident_detector(self):
         assert self.monitor.name == original_monitor_name
         assert self.monitor.config == original_monitor_config
 
+    def test_update_monitor_incident_detector_with_data_sources(self):
+        """Test updating a monitor detector with dataSources field (including existing slug)."""
+        data = {
+            "name": "Updated Name",
+            "dataSources": [
+                {
+                    "name": "Updated Monitor Name",
+                    "slug": self.monitor.slug,  # Keep the existing slug
+                    "config": {
+                        "checkin_margin": 1,
+                        "failure_issue_threshold": 1,
+                        "max_runtime": 30,
+                        "recovery_threshold": 1,
+                        "timezone": "UTC",
+                        "schedule": "0 0 * * 5",
+                        "schedule_type": "crontab",
+                    },
+                }
+            ],
+        }
+
+        response = self.get_success_response(
+            self.organization.slug,
+            self.detector.id,
+            **data,
+            status_code=200,
+            method="PUT",
+        )
+
+        assert response.data["name"] == "Updated Name"
+
+        self.monitor.refresh_from_db()
+        assert self.monitor.name == "Updated Monitor Name"
+        assert self.monitor.slug == "test-monitor"  # Slug unchanged
+        assert self.monitor.config["schedule"] == "0 0 * * 5"
+
     def test_delete_monitor_incident_detector(self):
         """Test deleting a monitor incident detector."""
         with outbox_runner():