On 4/23/07, Michael Bayer <[EMAIL PROTECTED]> wrote:
>
>
> On Apr 23, 2007, at 3:49 PM, Gaetan de Menten wrote:
>
> > Hello there,
> >
> > In a mapped object, is there any way to map a scalar attribute to an
> > arbitrary selectable/subquery?
> >
> > Jonathan Ellis demonstrated how to do that for relations on this page:
> > http://spyced.blogspot.com/2007/01/why-sqlalchemy-impresses-me.html
> >
> > I'd like to do that for scalars.
> >
> > I've thought about using a property returning a query, but this still
> > generates one query per user (my mapped object) and I need to do
> > everything in one pass.
> >
> > See attached file for an example of what I'd like to do.
> >
> > I've the feeling it might already be possible but I don't see how. If
> > it's not possible yet, do you have any pointer how I could implement
> > that?
> >
>
> there are probably three general ways to do what youre doing there.
Thanks for the quick answer!
> the oldest way is something i did in the zblog demo before SA 0.1 was
> released, which is that you map to the full query you want:
>
> s = select([users_table, func.sum(tags_table.c.score1 *
> tags_table.c.score2).label('score')],
> users_table.c.user_id==tags_table.c.user_id, group_by=[c for c in
> users_table.c])
>
> mapper(User, s)
>
> the effect above is that your func() becomes another ColumnProperty.
That's what I was looking for. So simple... and I didn't think of it. Damn...
> the next way is to do it almost the same as Jonathan's blog says to
> do it, except youd map the relation to some intermediary class like
> "Score", and then use AssociationProxy to apply the "scalar" property
> to the class.
I thought about something like this but it felt sooo hacky I
disregarded it quickly.
> I might put a built-in feature in sa for "scalar"
> properties that does this, automatically creating an anonymous class
> for the intermediary....so that would be the closest to a "scalar
> relation".
A built-in thing would be great, but I don't see the point in using an
intermediary class at all. Wouldn't it be possible to "just" skip the
"make an instance out of the value" part and be done with it (more
like in your first solution)? It's probably no as easy as I make it
sound but it really feels like a better solution.
--
Gaëtan de Menten
http://openhex.org
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