Make it so Django's timezone-aware DateTimeFields remain unchanged when Django resets the timezone. DateFields are not timezone-aware, so they do not need fixing.

Django converts the timezone of DateTimeFields to UTC when the field gets saved. This makes it so when you update a model which includes a DateTimeField, and your server is running in another timezone, the AuditLog will think you changed the timestamp, while it actually is the same time, but in another timezone. This commit adds a specific check in the model_instance_diff function for DateTimeField models (and any subclasses of it), which converts the old and new values to UTC before comparing them to see if they've actually changed. It also adds tests to see if the code works properly.

The extra test_setting (USE_TZ) is added because timezone support is disabled if it is not specified, this setting enables it.

src/auditlog/diff.py

@@ -1,7 +1,8 @@
 from __future__ import unicode_literals
 
 from django.core.exceptions import ObjectDoesNotExist
-from django.db.models import Model, NOT_PROVIDED
+from django.db.models import Model, NOT_PROVIDED, DateTimeField
+from django.utils import timezone
 from django.utils.encoding import smart_text
 
 
@@ -97,15 +98,26 @@ def model_instance_diff(old, new):
         fields = filtered_fields
 
     for field in fields:
-        try:
-            old_value = smart_text(getattr(old, field.name, None))
-        except ObjectDoesNotExist:
-            old_value = field.default if field.default is not NOT_PROVIDED else None
+        if isinstance(field, DateTimeField):
+            # DateTimeFields are timezone-aware, so we need to convert the field
+            # to its naive form before we can accuratly compare them for changes.
+            old_value = field.to_python(getattr(old, field.name, None))
+            if old_value is not None:
+                old_value = timezone.make_naive(old_value, timezone.utc)
 
-        try:
-            new_value = smart_text(getattr(new, field.name, None))
-        except ObjectDoesNotExist:
-            new_value = None
+            new_value = field.to_python(getattr(new, field.name, None))
+            if new_value is not None:
+                new_value = timezone.make_naive(new_value, timezone.utc)
+        else:
+            try:
+                old_value = smart_text(getattr(old, field.name, None))
+            except ObjectDoesNotExist:
+                old_value = field.default if field.default is not NOT_PROVIDED else None
+
+            try:
+                new_value = smart_text(getattr(new, field.name, None))
+            except ObjectDoesNotExist:
+                new_value = None
 
         if old_value != new_value:
             diff[field.name] = (smart_text(old_value), smart_text(new_value))

src/auditlog_tests/models.py

@@ -106,6 +106,18 @@ class AdditionalDataIncludedModel(models.Model):
         }
         return object_details
 
+
+class DateTimeFieldModel(models.Model):
+    """
+    A model with a DateTimeField, used to test DateTimeField
+    changes are detected properly.
+    """
+    label = models.CharField(max_length=100)
+    timestamp = models.DateTimeField()
+
+    history = AuditlogHistoryField()
+
+
 auditlog.register(SimpleModel)
 auditlog.register(AltPrimaryKeyModel)
 auditlog.register(ProxyModel)
@@ -115,3 +127,4 @@ auditlog.register(ManyRelatedModel.related.through)
 auditlog.register(SimpleIncludeModel, include_fields=['label'])
 auditlog.register(SimpleExcludeModel, exclude_fields=['text'])
 auditlog.register(AdditionalDataIncludedModel)
+auditlog.register(DateTimeFieldModel)

src/auditlog_tests/test_settings.py

@@ -24,3 +24,5 @@ DATABASES = {
 }
 
 ROOT_URLCONF = []
+
+USE_TZ = True

src/auditlog_tests/tests.py

@@ -4,11 +4,14 @@ from django.core.exceptions import ValidationError
 from django.db.models.signals import pre_save
 from django.http import HttpResponse
 from django.test import TestCase, RequestFactory
+from django.utils import timezone
+
 from auditlog.middleware import AuditlogMiddleware
 from auditlog.models import LogEntry
 from auditlog.registry import auditlog
 from auditlog_tests.models import SimpleModel, AltPrimaryKeyModel, ProxyModel, \
-    SimpleIncludeModel, SimpleExcludeModel, RelatedModel, ManyRelatedModel, AdditionalDataIncludedModel
+    SimpleIncludeModel, SimpleExcludeModel, RelatedModel, ManyRelatedModel, AdditionalDataIncludedModel, \
+    DateTimeFieldModel
 
 
 class SimpleModelTest(TestCase):
@@ -220,6 +223,68 @@ class AdditionalDataModelTest(TestCase):
                         msg="Related model's id is logged")
 
 
+class DateTimeFieldModelTest(TestCase):
+    """Tests if DateTimeField changes are recognised correctly"""
+
+    utc_plus_one = timezone.get_fixed_timezone(datetime.timedelta(hours=1))
+
+    def test_model_with_same_time(self):
+        timestamp = datetime.datetime(2017, 1, 10, 12, 0, tzinfo=timezone.utc)
+        dtm = DateTimeFieldModel(label='DateTimeField model', timestamp=timestamp)
+        dtm.save()
+        self.assertTrue(dtm.history.count() == 1, msg="There is one log entry")
+
+        # Change timestamp to same datetime and timezone
+        timestamp = datetime.datetime(2017, 1, 10, 12, 0, tzinfo=timezone.utc)
+        dtm.timestamp = timestamp
+        dtm.save()
+
+        # Nothing should have changed
+        self.assertTrue(dtm.history.count() == 1, msg="There is one log entry")
+
+    def test_model_with_different_timezone(self):
+        timestamp = datetime.datetime(2017, 1, 10, 12, 0, tzinfo=timezone.utc)
+        dtm = DateTimeFieldModel(label='DateTimeField model', timestamp=timestamp)
+        dtm.save()
+        self.assertTrue(dtm.history.count() == 1, msg="There is one log entry")
+
+        # Change timestamp to same datetime in another timezone
+        timestamp = datetime.datetime(2017, 1, 10, 13, 0, tzinfo=self.utc_plus_one)
+        dtm.timestamp = timestamp
+        dtm.save()
+
+        # Nothing should have changed
+        self.assertTrue(dtm.history.count() == 1, msg="There is one log entry")
+
+    def test_model_with_different_time(self):
+        timestamp = datetime.datetime(2017, 1, 10, 12, 0, tzinfo=timezone.utc)
+        dtm = DateTimeFieldModel(label='DateTimeField model', timestamp=timestamp)
+        dtm.save()
+        self.assertTrue(dtm.history.count() == 1, msg="There is one log entry")
+
+        # Change timestamp to another datetime in the same timezone
+        timestamp = datetime.datetime(2017, 1, 10, 13, 0, tzinfo=timezone.utc)
+        dtm.timestamp = timestamp
+        dtm.save()
+
+        # The time should have changed.
+        self.assertTrue(dtm.history.count() == 2, msg="There are two log entries")
+
+    def test_model_with_different_time_and_timezone(self):
+        timestamp = datetime.datetime(2017, 1, 10, 12, 0, tzinfo=timezone.utc)
+        dtm = DateTimeFieldModel(label='DateTimeField model', timestamp=timestamp)
+        dtm.save()
+        self.assertTrue(dtm.history.count() == 1, msg="There is one log entry")
+
+        # Change timestamp to another datetime and another timezone
+        timestamp = datetime.datetime(2017, 1, 10, 14, 0, tzinfo=self.utc_plus_one)
+        dtm.timestamp = timestamp
+        dtm.save()
+
+        # The time should have changed.
+        self.assertTrue(dtm.history.count() == 2, msg="There are two log entries")
+
+
 class UnregisterTest(TestCase):
     def setUp(self):
         auditlog.unregister(SimpleModel)