[fix] Correct handling of deferred fields when tracking changed fields

When a Device instance was loaded with deferred fields (e.g. using
QuerySet.only()), `_check_changed_fields()` incorrectly populated the
`_initial_<field>` attributes. Instead of storing the original value
retrieved from the database, it stored the field name itself.

This happened because the code iterated over `_changed_checked_fields`
and assigned the field name rather than the actual value coming from
`present_values`.

The logic has been updated to iterate over `present_values.items()`,
store the current database value in `_initial_<field>`, and then
restore the updated value on the instance.

This ensures change detection works correctly even when fields were
initially deferred.

Changelog (Bugfix):
Fixed incorrect initialization of `_initial_<field>` values when a
Device instance is loaded with deferred fields, which could break
change detection logic.

Author: pandafy

openwisp_controller/config/base/device.py

@@ -338,9 +338,9 @@ def _get_initial_values_for_checked_fields(self):
         if not present_values:
             return
         self.refresh_from_db(fields=present_values.keys())
-        for field in self._changed_checked_fields:
-            setattr(self, f"_initial_{field}", field)
-            setattr(self, field, present_values[field])
+        for field, value in present_values.items():
+            setattr(self, f"_initial_{field}", getattr(self, field))
+            setattr(self, field, value)
 
     def _check_name_changed(self):
         if self._initial_name == models.DEFERRED:

openwisp_controller/config/tests/test_device.py

@@ -541,6 +541,21 @@ def test_changed_checked_fields_no_duplicates(self):
         device.__init__()
         self.assertEqual(device._changed_checked_fields.count("last_ip"), 1)
 
+    def test_deferred_fields_populated_correctly(self):
+        device = self._create_device(
+            name="deferred-test",
+            last_ip="172.16.0.1",
+            management_ip="10.0.0.1",
+        )
+        # Load the instance with deferred fields omitted
+        device = Device.objects.only("id").get(pk=device.pk)
+        device.last_ip = "172.16.1.1"
+        # Populate initial values for checked fields
+        device._check_changed_fields()
+        # Ensure `_initial_<field>` contains the actual value, not the field name
+        self.assertEqual(getattr(device, "_initial_last_ip"), "172.16.0.1")
+        self.assertNotEqual(getattr(device, "_initial_last_ip"), "last_ip")
+
     def test_exceed_organization_device_limit(self):
         org = self._get_org()
         org.config_limits.device_limit = 1