On Mon, 2009-02-23 at 21:49 +0100, Torsten Bronger wrote:
> Hallöchen!
>
> Alex Gaynor writes:
>
> > On Mon, Feb 23, 2009 at 12:40 PM, sphogan <spho...@gmail.com> wrote:
[...]
> >> What I would like to be able to do is:
> >>
> >> Article.objects.select_related('comment_set').all()
> >>
> >> This seems like a somewhat simple case and there seems to be no
> >> mention of it in the documentation anywhere.
> >
> > That's not possible,
>
> By principle, or just because Django can't do it (yet)? The related
> feature request can be found at
> <http://code.djangoproject.com/ticket/2238>.
Actually, this is really just a disguised version of #5768, which is
talking about values() -- since that contains this problem plus the
presentation issue subtlety. Read the comments there for information
(I've repeated the basic hard part of this, below).
> It it, I refer to
> http://rubynugs.blogspot.com/2008/07/couple-of-stupid-things-about-djano.html
> where a Rails fan claims that Rails can do it.
>
> I don't know much about SQL so I really would like to know whether
> this could be realised.
It's possible, but it's not at all trivial. Firstly, there's a
representation issue for values(): there are multiple *_set rows
returned for any individual result in the original query. So it requires
a bunch more munging of results.
Secondly, there's the "don't shoot yourself in the foot" issue, in that
it's *very* easy to create a horribly inefficient query. The problem is
when you start querying for both foo_set and bar_set. If they return X
and Y results, respectively, for a given row, we need to make sure we
only get back O(X + Y) results from the database, not O(X * Y) results.
the simplest ways to construct the query for these multi-valued returns
leads to the second case, so extra care has to be put into the situation
with multiple multi-valued return results.
We've always said we'll put this in once there's an excellent patch that
avoids the second problem and has a decent return format for values()
Hopefully that should put to rest all the debate in this thread about
what's going on. We'll do, but somebody has to write the patch and it's
not trivial. (That somebody might be me one day, but I also about a few
hundred other things to work on in Django, so it's prioritised
accordingly).
Regards,
Malcolm
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