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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def by_filename(cls, session, release, filename):
 """ Get a release file for a given release and a given filename. :param session: SQLAlchemy session :type session: :class:`sqlalchemy.Session` :param release: release :type release: :class:`pyshop.models.Release` :param filename: filename of the release file :type filename: unicode :return: release file :rtype: :class:`pyshop.models.ReleaseFile` """

        return cls.first(session,
                         where=(ReleaseFile.release_id == release.id,
                                ReleaseFile.filename == filename,
                                ))