Fix AttributeError for None or missing nested fields in __propagate_dump_mode__ function

Author: Sergey Vilgelm

marshmallow_objects/models.py

@@ -157,7 +157,9 @@ def __propagate_dump_mode__(self, value):
         self.__dump_mode__ = value
         for name, field in self.__schema__.fields.items():
             if isinstance(field, fields.Nested):
-                getattr(self, name).__propagate_dump_mode__(value)
+                nested = getattr(self, name, None)
+                if nested is not None and nested != marshmallow.missing:
+                    nested.__propagate_dump_mode__(value)
 
     @contextlib.contextmanager
     def __dump_mode_on__(self):

tests/test_models.py

@@ -538,6 +538,7 @@ class MissingPerson(marshmallow.Model):
 class MissingCompany(marshmallow.Model):
     name = marshmallow.fields.String()
     owner = marshmallow.NestedModel(MissingPerson)
+    hr = marshmallow.NestedModel(MissingPerson, allow_none=True)
 
 
 class TestMissingFields(unittest.TestCase):