def create(cls, mp, part_number, stream=None, **kwargs):
        """Create a new part object in a multipart object."""
        if part_number < 0 or part_number > mp.last_part_number:
            raise MultipartInvalidPartNumber()

        with db.session.begin_nested():
            obj = cls(
                multipart=mp,
                part_number=part_number,
            )
            db.session.add(obj)
        if stream:
            obj.set_contents(stream, **kwargs)
        return obj