Fixed inconsistency handling None values field attrs

fixes hmarr/mongoengine#505

mongoengine/base.py

@@ -833,13 +833,6 @@ class BaseDocument(object):
                 if hasattr(self, '_changed_fields'):
                     self._mark_as_changed(name)
 
-        # Handle None values for required fields
-        if value is None and name in getattr(self, '_fields', {}):
-            self._data[name] = value
-            if hasattr(self, '_changed_fields'):
-                self._mark_as_changed(name)
-            return
-
         if not self._created and name in self._meta.get('shard_key', tuple()):
             from queryset import OperationError
             raise OperationError("Shard Keys are immutable. Tried to update %s" % name)

mongoengine/fields.py

@@ -369,7 +369,7 @@ class ComplexDateTimeField(StringField):
         return self._convert_from_string(data)
 
     def __set__(self, instance, value):
-        value = self._convert_from_datetime(value)
+        value = self._convert_from_datetime(value) if value else value
         return super(ComplexDateTimeField, self).__set__(instance, value)
 
     def validate(self, value):