Add failing test for deferred attributes.

model_utils/tracker.py

@@ -107,8 +107,7 @@ class FieldInstanceTracker(object):
                 field_tracker = FileDescriptorTracker(field_obj.field)
                 setattr(self.instance.__class__, field, field_tracker)
             else:
-                field_tracker = DeferredAttributeTracker(
-                    field_obj.field_name, None)
+                field_tracker = DeferredAttributeTracker(field, type(self.instance))
                 setattr(self.instance.__class__, field, field_tracker)
 
 

tests/models.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals, absolute_import
 
 from django.db import models
+from django.db.models.query_utils import DeferredAttribute
 from django.db.models import Manager
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.translation import ugettext_lazy as _
@@ -331,3 +332,37 @@ class CustomSoftDelete(SoftDeletableModel):
     is_read = models.BooleanField(default=False)
 
     objects = CustomSoftDeleteManager()
+
+
+class StringyDescriptor(object):
+    """
+    Descriptor that returns a string version of the underlying integer value.
+    """
+    def __init__(self, name):
+        self.name = name
+
+    def __get__(self, obj, cls=None):
+        if obj is None:
+            return self
+        if self.name in obj.get_deferred_fields():
+            # This queries the database, and sets the value on the instance.
+            DeferredAttribute(field_name=self.name, model=cls).__get__(obj, cls)
+        return str(obj.__dict__[self.name])
+
+    def __set__(self, obj, value):
+        obj.__dict__[self.name] = int(value)
+
+
+class CustomDescriptorField(models.IntegerField):
+    def contribute_to_class(self, cls, name, **kwargs):
+        super(CustomDescriptorField, self).contribute_to_class(cls, name, **kwargs)
+        setattr(cls, name, StringyDescriptor(name))
+
+
+class ModelWithCustomDescriptor(models.Model):
+    custom_field = CustomDescriptorField()
+    tracked_custom_field = CustomDescriptorField()
+    regular_field = models.IntegerField()
+    tracked_regular_field = models.IntegerField()
+
+    tracker = FieldTracker(fields=['tracked_custom_field', 'tracked_regular_field'])

tests/test_models/test_deferred_fields.py

@@ -0,0 +1,53 @@
+from __future__ import unicode_literals
+
+from django.test import TestCase
+
+from tests.models import ModelWithCustomDescriptor
+
+
+class CustomDescriptorTests(TestCase):
+    def setUp(self):
+        self.instance = ModelWithCustomDescriptor.objects.create(
+            custom_field='1',
+            tracked_custom_field='1',
+            regular_field=1,
+            tracked_regular_field=1,
+        )
+
+    def test_custom_descriptor_works(self):
+        instance = self.instance
+        self.assertEqual(instance.custom_field, '1')
+        self.assertEqual(instance.__dict__['custom_field'], 1)
+        self.assertEqual(instance.regular_field, 1)
+        instance.custom_field = 2
+        self.assertEqual(instance.custom_field, '2')
+        self.assertEqual(instance.__dict__['custom_field'], 2)
+        instance.save()
+        intance = ModelWithCustomDescriptor.objects.get(pk=instance.pk)
+        self.assertEqual(instance.custom_field, '2')
+        self.assertEqual(instance.__dict__['custom_field'], 2)
+
+    def test_deferred(self):
+        instance = ModelWithCustomDescriptor.objects.only('id').get(
+            pk=self.instance.pk)
+        self.assertIn('custom_field', instance.get_deferred_fields())
+        self.assertEqual(instance.custom_field, '1')
+        self.assertNotIn('custom_field', instance.get_deferred_fields())
+        self.assertEqual(instance.regular_field, 1)
+        self.assertEqual(instance.tracked_custom_field, '1')
+        self.assertEqual(instance.tracked_regular_field, 1)
+
+        self.assertFalse(instance.tracker.has_changed('tracked_custom_field'))
+        self.assertFalse(instance.tracker.has_changed('tracked_regular_field'))
+
+        instance.tracked_custom_field = 2
+        instance.tracked_regular_field = 2
+        self.assertTrue(instance.tracker.has_changed('tracked_custom_field'))
+        self.assertTrue(instance.tracker.has_changed('tracked_regular_field'))
+        instance.save()
+
+        instance = ModelWithCustomDescriptor.objects.get(pk=instance.pk)
+        self.assertEqual(instance.custom_field, '1')
+        self.assertEqual(instance.regular_field, 1)
+        self.assertEqual(instance.tracked_custom_field, '2')
+        self.assertEqual(instance.tracked_regular_field, 2)