Improve coverage.

model_utils/tracker.py

@@ -40,10 +40,7 @@ class DescriptorWrapper(object):
         if instance is None:
             return self
         was_deferred = self.field_name in instance.get_deferred_fields()
-        if self.descriptor:
-            value = self.descriptor.__get__(instance, owner)
-        else:
-            value = instance.__dict__[self.field_name]
+        value = self.descriptor.__get__(instance, owner)
         if was_deferred:
             tracker_instance = getattr(instance, self.tracker_attname)
             tracker_instance.saved_data[self.field_name] = deepcopy(value)
@@ -53,12 +50,10 @@ class DescriptorWrapper(object):
     def cls_for_descriptor(descriptor):
         has_set = hasattr(descriptor, '__set__')
         has_del = hasattr(descriptor, '__delete__')
-        if has_set and has_del:
+        if has_del:
             return FullDescriptorWrapper
         elif has_set:
             return SettableDescriptorWrapper
-        elif has_del:
-            return DeleteableDescriptorWrapper
         else:
             return DescriptorWrapper
 
@@ -73,20 +68,12 @@ class SettableDescriptorWrapper(DescriptorWrapper):
         return self.descriptor.__set__(instance, value)
 
 
-class DeleteableDescriptorWrapper(DescriptorWrapper):
-    """
-    Descriptor wrapper for descriptors with a __delete__ method.
-
-    This should not be used for descriptors
-    """
-    def __delete__(self, instance):
-        self.descriptor.__delete__(instance)
-
-
-class FullDescriptorWrapper(SettableDescriptorWrapper, DeleteableDescriptorWrapper):
+class FullDescriptorWrapper(SettableDescriptorWrapper):
     """
     Wrapper for descriptors with all three descriptor methods.
     """
+    def __delete__(self, obj):
+        self.descriptor.__delete__(obj)
 
 
 class FieldInstanceTracker(object):
@@ -161,10 +148,7 @@ class FieldInstanceTracker(object):
 
         self.instance._deferred_fields = self.instance.get_deferred_fields()
         for field in self.instance._deferred_fields:
-            if django.VERSION >= (1, 10):
-                field_obj = getattr(self.instance.__class__, field)
-            else:
-                field_obj = self.instance.__class__.__dict__.get(field)
+            field_obj = self.instance.__class__.__dict__.get(field)
             if isinstance(field_obj, FileDescriptor):
                 field_tracker = FileDescriptorTracker(field_obj.field)
                 setattr(self.instance.__class__, field, field_tracker)
@@ -199,10 +183,7 @@ class FieldTracker(object):
         self.fields = set(self.fields)
         if django.VERSION >= (1, 10):
             for field_name in self.fields:
-                if django.VERSION >= (1, 10):
-                    descriptor = getattr(sender, field_name)
-                else:
-                    descriptor = sender.__dict__.get(field_name)
+                descriptor = getattr(sender, field_name)
                 wrapper_cls = DescriptorWrapper.cls_for_descriptor(descriptor)
                 wrapped_descriptor = wrapper_cls(field_name, descriptor, self.attname)
                 setattr(sender, field_name, wrapped_descriptor)

tests/models.py

@@ -352,6 +352,9 @@ class StringyDescriptor(object):
     def __set__(self, obj, value):
         obj.__dict__[self.name] = int(value)
 
+    def __delete__(self, obj):
+        del obj.__dict__[self.name]
+
 
 class CustomDescriptorField(models.IntegerField):
     def contribute_to_class(self, cls, name, **kwargs):

tests/test_models/test_deferred_fields.py

@@ -58,3 +58,14 @@ class CustomDescriptorTests(TestCase):
         self.assertEqual(instance.regular_field, 1)
         self.assertEqual(instance.tracked_custom_field, '2')
         self.assertEqual(instance.tracked_regular_field, 2)
+
+        instance = ModelWithCustomDescriptor.objects.only('id').get(pk=instance.pk)
+        if django.VERSION >= (1, 10):
+            # This fails on 1.8 and 1.9, which is a bug in the deferred field
+            # implementation on those versions.
+            instance.tracked_custom_field = 3
+            self.assertEqual(instance.tracked_custom_field, '3')
+            self.assertTrue(instance.tracker.has_changed('tracked_custom_field'))
+            del instance.tracked_custom_field
+            self.assertEqual(instance.tracked_custom_field, '2')
+            self.assertFalse(instance.tracker.has_changed('tracked_custom_field'))