If values cant be compared mark as changed (#287)

docs/changelog.rst

@@ -4,6 +4,7 @@ Changelog
 
 Changes in 0.8.X
 ================
+- If values cant be compared mark as changed (#287)
 - Ensure as_pymongo() and to_json honour only() and exclude() (#293)
 - Document serialization uses field order to ensure a strict order is set (#296)
 - DecimalField now stores as float not string (#289)

mongoengine/base/fields.py

@@ -81,13 +81,16 @@ class BaseField(object):
     def __set__(self, instance, value):
         """Descriptor for assigning a value to a field in a document.
         """
-        changed = False
+        if instance._initialised:
+            try:
                 if (self.name not in instance._data or
                    instance._data[self.name] != value):
-            changed = True
-            instance._data[self.name] = value
-        if changed and instance._initialised:
                     instance._mark_as_changed(self.name)
+            except:
+                # Values cant be compared eg: naive and tz datetimes
+                # So mark it as changed
+                instance._mark_as_changed(self.name)
+        instance._data[self.name] = value
 
     def error(self, message="", errors=None, field_name=None):
         """Raises a ValidationError.

tests/fields/fields.py

@@ -409,6 +409,27 @@ class FieldTest(unittest.TestCase):
         log.time = '1pm'
         self.assertRaises(ValidationError, log.validate)
 
+    def test_datetime_tz_aware_mark_as_changed(self):
+        from mongoengine import connection
+
+        # Reset the connections
+        connection._connection_settings = {}
+        connection._connections = {}
+        connection._dbs = {}
+
+        connect(db='mongoenginetest', tz_aware=True)
+
+        class LogEntry(Document):
+            time = DateTimeField()
+
+        LogEntry.drop_collection()
+
+        LogEntry(time=datetime.datetime(2013, 1, 1, 0, 0, 0)).save()
+
+        log = LogEntry.objects.first()
+        log.time = datetime.datetime(2013, 1, 1, 0, 0, 0)
+        self.assertEqual(['time'], log._changed_fields)
+
     def test_datetime(self):
         """Tests showing pymongo datetime fields handling of microseconds.
         Microseconds are rounded to the nearest millisecond and pre UTC