Fix ValueError when calling prefetch_related for tracked ForeignKey fields

AUTHORS.rst

@@ -45,6 +45,7 @@
 | Jannis Leidel <[email protected]>
 | Javier Garcia Sogo <[email protected]>
 | Jeff Elmore <[email protected]>
+| Joe Riddle <[email protected]>
 | John Vandenberg <[email protected]>
 | Jonathan Sundqvist <[email protected]>
 | João Amaro <[email protected]>

CHANGES.rst

@@ -12,6 +12,7 @@ To be released
 - Drop support for `Python 3.7` (GH-#545)
 - Swedish translation (GH-#561)
 - Use proper column name instead of attname (GH-#573)
+- Fix `ValueError` when calling `prefetch_related` for tracked `ForeignKey` fields (Fixes GH-433)
 
 4.3.1 (2022-11-15)
 ------------------

model_utils/tracker.py

@@ -103,6 +103,9 @@ def __set__(self, instance, value):
         else:
             instance.__dict__[self.field_name] = value
 
+    def __getattr__(self, attr):
+        return getattr(self.descriptor, attr)
+
     @staticmethod
     def cls_for_descriptor(descriptor):
         if hasattr(descriptor, '__delete__'):

tests/test_fields/test_field_tracker.py

@@ -538,6 +538,29 @@ def test_custom_without_id(self):
         self.assertCurrent(fk=self.instance.fk_id)
 
 
+class FieldTrackerForeignKeyPrefetchRelatedTests(FieldTrackerTestCase):
+    """Test that using `prefetch_related` on a tracked field does not raise a ValueError."""
+
+    fk_class = Tracked
+    tracked_class = TrackedFK
+
+    def setUp(self):
+        model_tracked = self.fk_class.objects.create(name="", number=0)
+        self.instance = self.tracked_class.objects.create(fk=model_tracked)
+
+    def test_default(self):
+        self.tracker = self.instance.tracker
+        self.assertIsNotNone(list(self.tracked_class.objects.prefetch_related("fk")))
+
+    def test_custom(self):
+        self.tracker = self.instance.custom_tracker
+        self.assertIsNotNone(list(self.tracked_class.objects.prefetch_related("fk")))
+
+    def test_custom_without_id(self):
+        self.tracker = self.instance.custom_tracker_without_id
+        self.assertIsNotNone(list(self.tracked_class.objects.prefetch_related("fk")))
+
+
 class FieldTrackerTimeStampedTests(FieldTrackerTestCase):
 
     fk_class = Tracked