如何在sqlalchemy中生成此查询?

我想在sqlalchemy中生成这个查询。数据库中存在表“demande”。有一个子查询使用generate\ u series函数生成时间步。

SELECT
    timesteps.timestep AS timestep, d.count AS count
FROM
    (SELECT
        DATE_TRUNC('hour',date_demande) AS timestep,
        COUNT(id) AS count
     FROM
        demande
     GROUP BY
        timestep
    ) AS d

RIGHT OUTER JOIN
    (SELECT
        timestep
     FROM
        generate_series('2010-01-01 00:00:00'::timestamp,
                        '2010-01-01 23:59:59'::timestamp,
                        '1 hour'::interval) AS timestep
     ) AS timesteps
  ON d.timestep = timesteps.timestep

ORDER BY timestep;

我试过这个:

stmt = session.query(
        func.
            generate_series(
                datetime.datetime(2010,1,1,0,0,0),
                datetime.datetime(2010,1,1,23,59,59),
                cast('1 hour',Interval())).
            label('timestep')
        ).subquery()
print stmt
q = session.query(
        stmt.c.timestep,
        func.count(Demande.id)).
    outerjoin((Demande, grouped==stmt.c.timestep)).
    group_by(stmt.c.timestep)
print q

q = session.query(
        func.count(Demande.id),
        stmt.c.timestep).
    outerjoin((stmt, grouped==stmt.c.timestep)).
    group_by(stmt.c.timestep)

由于sqlalchemy中没有正确的外部联接,所以我只想找到一种方法,将子查询作为第一个表,将“demande”表作为第二个表。这样我就可以使用左外连接