Add support for deferred fields in the FieldTracker

AUTHORS.rst

@@ -16,6 +16,7 @@ Jannis Leidel <jannis@leidel.info>
 Javier García Sogo <jgsogo@gmail.com>
 Jeff Elmore <jeffelmore.org>
 Keryn Knight <kerynknight.com>
+Michael van Tellingen <michaelvantellingen@gmail.com>
 Mikhail Silonov <silonov.pro>
 Patryk Zawadzki <patrys@room-303.com>
 Paul McLanahan <paul@mclanahan.net>

model_utils/tests/tests.py

@@ -1436,7 +1436,19 @@ class FieldTrackerTests(FieldTrackerTestCase, FieldTrackerCommonTests):
         self.instance.name = 'new age'
         self.instance.number = 1
         self.instance.save()
-        items = list(self.tracked_class.objects.only('name').all())
+        item = list(self.tracked_class.objects.only('name').all())[0]
+        self.assertTrue(item.tracker.deferred_fields)
+
+        self.assertEqual(item.tracker.previous('number'), None)
+        self.assertTrue('number' in item.tracker.deferred_fields)
+
+        self.assertEqual(item.number, 1)
+        self.assertTrue('number' not in item.tracker.deferred_fields)
+        self.assertEqual(item.tracker.previous('number'), 1)
+        self.assertFalse(item.tracker.has_changed('number'))
+
+        item.number = 2
+        self.assertTrue(item.tracker.has_changed('number'))
 
 
 class FieldTrackedModelCustomTests(FieldTrackerTestCase,

model_utils/tracker.py

@@ -4,6 +4,7 @@ from copy import deepcopy
 
 from django.db import models
 from django.core.exceptions import FieldError
+from django.db.models.query_utils import DeferredAttribute
 
 
 class FieldInstanceTracker(object):
@@ -11,6 +12,7 @@ class FieldInstanceTracker(object):
         self.instance = instance
         self.fields = fields
         self.field_map = field_map
+        self.init_deferred_fields()
 
     def get_field_value(self, field):
         return getattr(self.instance, self.field_map[field])
@@ -30,7 +32,14 @@ class FieldInstanceTracker(object):
     def current(self, fields=None):
         """Returns dict of current values for all tracked fields"""
         if fields is None:
-            fields = self.fields
+            if self.deferred_fields:
+                fields = [
+                    field for field in self.fields
+                    if field not in self.deferred_fields
+                ]
+            else:
+                fields = self.fields
+
         return dict((f, self.get_field_value(f)) for f in fields)
 
     def has_changed(self, field):
@@ -52,6 +61,30 @@ class FieldInstanceTracker(object):
             if self.has_changed(field)
         )
 
+    def init_deferred_fields(self):
+        self.deferred_fields = []
+        if not self.instance._deferred:
+            return
+
+        class DeferredAttributeTracker(DeferredAttribute):
+            def __get__(field, instance, owner):
+                data = instance.__dict__
+                if data.get(field.field_name, field) is field:
+                    self.deferred_fields.remove(field.field_name)
+                    value = super(DeferredAttributeTracker, field).__get__(
+                        instance, owner)
+                    self.saved_data[field.field_name] = deepcopy(value)
+                return data[field.field_name]
+
+        for field in self.fields:
+            field_obj = self.instance.__class__.__dict__.get(field)
+            if isinstance(field_obj, DeferredAttribute):
+                self.deferred_fields.append(field)
+
+                field_tracker = DeferredAttributeTracker(
+                    field_obj.field_name, None)
+                setattr(self.instance.__class__, field, field_tracker)
+
 
 class FieldTracker(object):