fix(uptime): Prevent Environment.DoesNotExist on monitor update

src/sentry/uptime/endpoints/validators.py

@@ -417,14 +417,16 @@ def update(self, instance: Detector, data):
         )
 
         if "environment" in data:
+            env_name = data["environment"]
+        else:
+            env_name = instance.config["environment"]
+
+        if env_name is None:
+            environment = None
+        else:
             environment = Environment.get_or_create(
                 project=self.context["project"],
-                name=data["environment"],
-            )
-        else:
-            environment = Environment.objects.get(
-                projects=self.context["project"],
-                name=instance.config["environment"],
+                name=env_name,
             )
 
         if "mode" in data:

tests/sentry/uptime/endpoints/test_project_uptime_alert_details.py

@@ -125,6 +125,29 @@ def test_enviroment(self) -> None:
         assert detector.name == "test"
         assert detector.config.get("environment") == "uptime-prod"
 
+    def test_update_without_environment_when_monitor_has_none(self) -> None:
+        """Regression test: updating a monitor created without an environment
+        must not raise Environment.DoesNotExist when environment is omitted from
+        the PUT payload."""
+        detector = self.create_uptime_detector()
+        # Simulate a monitor that was created without an environment by clearing
+        # the environment from the config (mirrors the pre-existing bug scenario).
+        detector.config["environment"] = None
+        detector.save(update_fields=["config"])
+        assert detector.config.get("environment") is None
+
+        resp = self.get_success_response(
+            self.organization.slug,
+            detector.linked_project.slug,
+            detector.id,
+            name="updated-name",
+        )
+        detector.refresh_from_db()
+        assert resp.data == serialize(detector, self.user, UptimeDetectorSerializer())
+        assert detector.name == "updated-name"
+        # Environment should remain None
+        assert detector.config.get("environment") is None
+
     def test_user(self) -> None:
         detector = self.create_uptime_detector()