Apply defaults to fields with None value at 'set' time.

mongoengine/base/fields.py

@@ -82,6 +82,12 @@ def __get__(self, instance, owner):
     def __set__(self, instance, value):
         """Descriptor for assigning a value to a field in a document.
         """
+        if value is None:
+            value = self.default
+            # Allow callable default values
+            if callable(value):
+                value = value()
+
         if instance._initialised:
             try:
                 if (self.name not in instance._data or

tests/fields/fields.py

@@ -44,6 +44,19 @@ class Person(Document):
         self.assertEqual(person._fields['age'].help_text, "Your real age")
         self.assertEqual(person._fields['userid'].verbose_name, "User Identity")
 
+        class Person2(Document):
+            created = DateTimeField(default=datetime.datetime.utcnow)
+
+        person = Person2()
+        date1 = person.created
+        date2 = person.created
+        self.assertEqual(date1, date2)
+
+        person = Person2(created=None)
+        date1 = person.created
+        date2 = person.created
+        self.assertEqual(date1, date2)
+
     def test_required_values(self):
         """Ensure that required field constraints are enforced.
         """