@classmethod
def exists(cls, session, tar_sum, filename):
    """ Retrieve the file that has:

        - a particular tarball md5sum
        - a particular filename
        """
    query = session.query(cls).filter(cls.tar_sum == tar_sum).filter(cls.
        filename == filename).order_by(cls.pkg_name, cls.tar_file, cls.filename
        )
    return query.first()
