fix complex datetime field invalid string set

tests/fields/test_complex_datetime_field.py

@@ -4,6 +4,8 @@ import itertools
 import math
 import re
 
+import pytest
+
 from mongoengine import *
 
 from tests.utils import MongoDBTestCase
@@ -191,3 +193,18 @@ class ComplexDateTimeFieldTest(MongoDBTestCase):
 
         fetched_log = Log.objects.with_id(log.id)
         assert fetched_log.timestamp >= NOW
+
+    def test_setting_bad_value_does_not_raise_unless_validate_is_called(self):
+        # test regression of #2253
+
+        class Log(Document):
+            timestamp = ComplexDateTimeField()
+
+        Log.drop_collection()
+
+        log = Log(timestamp="garbage")
+        with pytest.raises(ValidationError):
+            log.validate()
+
+        with pytest.raises(ValidationError):
+            log.save()