Hi,
Thanks for the reply (and sorry for my late response!).
I am indeed on a 0.7 build. Could you go into a little more detail about
how correlate is used? The documentation for that method seems a little
vague to me.
Thanks!
On Friday, January 25, 2013 1:03:00 AM UTC-5, Michael Bayer wrote:
>
>
> On Jan 24, 2013, at 11:03 PM, Dave Pedu wrote:
>
> Hi all,
>
> I am trying to run a query like this one, using sqlalchemy:
>
> SELECT
> t.`ForumID`,
> ( SELECT `ID` FROM `posts` p WHERE `ThreadID` = t.`ID` ORDER BY p.`Date`
> DESC LIMIT 1 ) as `LastPost`
> FROM `threads` t
> WHERE t.`Deleted` = 0
>
>
> I am unsure how to achieve the subquery that comes out as 'LastPost'.
>
> Closest I have gotten is this:
>
> lastPostQ = s.query(db.Post.ID).filter(db.Post.ThreadID==db.Thread.ID
> ).order_by(desc(db.Post.Date)).limit(1).label("LastPost")
> q = s.query(
> db.Thread.ForumID,
> lastPostQ
> ).\
> join(db.Forum, db.Forum.ID==db.Thread.ForumID).\
> join(db.ForumPermission, db.ForumPermission.ForumID==db.Thread.ForumID).\
> filter(db.Thread.Deleted==0)
>
>
> Which generates the following SQL:
>
> SELECT
> threads."ForumID" AS "threads_ForumID",
> (SELECT posts."ID" FROM posts, threads WHERE posts."ThreadID" =
> threads."ID" ORDER BY posts."Date" DESC LIMIT :param_1) AS "LastPost"
> FROM
> threads
> WHERE
> threads."Deleted" = :Deleted_1
>
>
> Which is close, but the subqueries select statement is selecting "FROM
> posts, threads" where it should only be "FROM posts".
>
> I don't know how to procede from here. A member on IRC suggested
> using correlate() but I am unsure as how or where to use it, as the
> documentation was unclear.
>
>
> if you're on 0.7 then the correlation used by query() is not automatic,
> you need to call query = query.correlate(Thread) on that subquery so it
> knows that "Thread" as a FROM object will be supplied from an enclosing
> query. In 0.8 the correlation is automatic by default with Query, the same
> way as it works with a select() construct.
>
>
>
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