Fixes #20912: Clear ModuleBay parent when module assignment removed

When a ModuleBay's module field is cleared (returning it to device-level),
the parent field was not being updated, leaving stale MPTT tree data.

The save() method only set parent when module was present, but failed to
clear it when module was removed. Now explicitly sets parent to None when
module is cleared.

netbox/dcim/models/device_components.py

@@ -1222,6 +1222,8 @@ class ModuleBay(ModularComponentModel, TrackingModelMixin, MPTTModel):
     def save(self, *args, **kwargs):
         if self.module:
             self.parent = self.module.module_bay
+        else:
+            self.parent = None
         super().save(*args, **kwargs)
 
 

netbox/dcim/tests/test_models.py

@@ -841,6 +841,32 @@ class ModuleBayTestCase(TestCase):
         nested_bay = module.modulebays.get(name='SFP A-21')
         self.assertEqual(nested_bay.label, 'A-21')
 
+    @tag('regression')  # #20912
+    def test_module_bay_parent_cleared_when_module_removed(self):
+        """Test that the parent field is properly cleared when a module bay's module assignment is removed"""
+        device = Device.objects.first()
+        manufacturer = Manufacturer.objects.first()
+        module_type = ModuleType.objects.create(manufacturer=manufacturer, model='Test Module Type')
+        bay1 = ModuleBay.objects.create(device=device, name='Test Bay 1')
+        bay2 = ModuleBay.objects.create(device=device, name='Test Bay 2')
+
+        # Install a module in bay1
+        module1 = Module.objects.create(device=device, module_bay=bay1, module_type=module_type)
+
+        # Assign bay2 to module1 and verify parent is now set to bay1 (module1's bay)
+        bay2.module = module1
+        bay2.save()
+        bay2.refresh_from_db()
+        self.assertEqual(bay2.parent, bay1)
+        self.assertEqual(bay2.module, module1)
+
+        # Clear the module assignment (return bay2 to device level) Verify parent is cleared
+        bay2.module = None
+        bay2.save()
+        bay2.refresh_from_db()
+        self.assertIsNone(bay2.parent)
+        self.assertIsNone(bay2.module)
+
 
 class CableTestCase(TestCase):