Upgrade to 2.0.0 breaks CI/CD with ValueError

Hi there, love the library!

We recently spotted an error in our CI/CD pipeline which was occurring after this package upgraded to 2.0.0.post0, the issue occurs when a OneOfSchema with fields.Nested is used on the line (in marshmallow/fields.py:451):

        ret, errors = schema.dump(nested_obj, many=self.many,
                update_fields=not self.__updated_fields)

The function schema.dump appears to be returning an empty list object instead of a MarshalResult object resulting in:

ValueError: not enough values to unpack (expected 2, got 0)

Hotfix:

Downgrade to 1.0.5

[lafrech]

True.

@sloria, do you think 2.0.0 releases should be removed and marshmallow-oneofschema be kept in beta until MA3 is out?

[sloria]

Releasing 2.x as a pre-release doesn’t seem quite right. 2.0 is meant to be a stable release even though it targets marshmallow 3.x. The release lines should be relatively independent. Also, we’re already pinning marshmallow-oneofschema to marshmallow>=3.0.0rc6, so you should get incompatible versions.