def by_version(cls, session, package_name, version):
        """
        Get release for a given version.

        :param session: SQLAlchemy session
        :type session: :class:`sqlalchemy.Session`

        :param package_name: package name
        :type package_name: unicode

        :param version: version
        :type version: unicode

        :return: release instance
        :rtype: :class:`pyshop.models.Release`
        """
        return cls.first(session,
                         join=(Package,),
                         where=((Package.name == package_name),
                                (cls.version == version)))