Catch deferred attribute exception (#367)

CHANGES.rst

@@ -3,6 +3,7 @@ CHANGES
 
 master (unreleased)
 -------------------
+- Catch `AttributeError` for deferred abstract fields, fixes GH-331.
 - Update documentation to explain usage of `timeframed` model manager, fixes GH-118
 - Honor `OneToOneField.parent_link=False`.
 - Fix handling of deferred attributes on Django 1.10+, fixes GH-278
@@ -14,7 +15,7 @@ master (unreleased)
 - Support `reversed` for all kinds of `Choices` objects, fixes GH-309
 - Fix Model instance non picklable GH-330
 - Fix patched `save` in FieldTracker
-- Upgrades test requirements (pytest, pytest-django, pytest-cov) and 
+- Upgrades test requirements (pytest, pytest-django, pytest-cov) and
   skips tox test with Python 3.5 and Django (trunk)
 
 3.1.2 (2018.05.09)

model_utils/tracker.py

@@ -40,7 +40,10 @@ class DescriptorWrapper(object):
         if instance is None:
             return self
         was_deferred = self.field_name in instance.get_deferred_fields()
-        value = self.descriptor.__get__(instance, owner)
+        try:
+            value = self.descriptor.__get__(instance, owner)
+        except AttributeError:
+            value = self.descriptor
         if was_deferred:
             tracker_instance = getattr(instance, self.tracker_attname)
             tracker_instance.saved_data[self.field_name] = deepcopy(value)

tests/models.py

@@ -224,6 +224,13 @@ class FeaturedManager(models.Manager):
         return ByAuthorQuerySet(self.model, **kwargs).filter(feature=True)
 
 
+class AbstractTracked(models.Model):
+    number = 1
+
+    class Meta:
+        abstract = True
+
+
 class Tracked(models.Model):
     name = models.CharField(max_length=20)
     number = models.IntegerField()
@@ -240,6 +247,14 @@ class TrackedFK(models.Model):
     custom_tracker_without_id = FieldTracker(fields=['fk'])
 
 
+class TrackedAbstract(AbstractTracked):
+    name = models.CharField(max_length=20)
+    number = models.IntegerField()
+    mutable = MutableField(default=None)
+
+    tracker = FieldTracker()
+
+
 class TrackedNotDefault(models.Model):
     name = models.CharField(max_length=20)
     number = models.IntegerField()

tests/test_fields/test_field_tracker.py

@@ -8,7 +8,7 @@ from model_utils import FieldTracker
 from model_utils.tracker import DescriptorWrapper
 from tests.models import (
     Tracked, TrackedFK, InheritedTrackedFK, TrackedNotDefault, TrackedNonFieldAttr, TrackedMultiple,
-    InheritedTracked, TrackedFileField,
+    InheritedTracked, TrackedFileField, TrackedAbstract,
     ModelTracked, ModelTrackedFK, ModelTrackedNotDefault, ModelTrackedMultiple, InheritedModelTracked,
 )
 
@@ -785,3 +785,8 @@ class InheritedModelTrackerTests(ModelTrackerTests):
         self.name2 = 'test'
         self.assertEqual(self.tracker.previous('name2'), None)
         self.assertTrue(self.tracker.has_changed('name2'))
+
+
+class AbstractModelTrackerTests(FieldTrackerTestCase):
+
+    tracked_class = TrackedAbstract