On Fri, 6 Mar 2009, Malcolm Tredinnick wrote:
> > Bob.objects.filter(foo=myFoo).filter(blah__in=myBlahs)
> Seems like the best (and obvious) way to me.
Gotcha.
> Yes it does. As written, your models have no ordering requirements. That
> complete lack of constraint is preserved perfectly. :-)
True ;)
> Precisely. If you want ordering, you have to specify it. The database is
> in no way obliged to return rows even in the order they are stored on
> disk, so if you're relying on that, you are making an error.
Fair enough, that's what I expected as I know that SQL IN isn't guaranteed
to preserve anything. I thought that there might be some chance that
Django was doing something on top of that, but doubted it (and turned out
to be right).
Here's a followup though. To go back to my original model descriptions:
class Foo:
asdf = models.CharField()
class Blah:
qwerty = models.CharField()
class Bob:
foo = models.ForeignKey(Foo)
blah = models.ForeignKey(Blah)
Suppose that for a given Foo (say foo.id = 42), I want the Bobs associated
with the entire set of Blahs. In SQL I could do something like this
(might not be exactly right, but should demonstrate the idea):
select * from app_bob,app_blah where app_bob.blah_id=app_blah.id and
app_foo_id=42;
I'd imagine this would be faster than going through the IN directive
(particularly considering that the entire set of Blahs will typically be
50k-250k in length). I know that I can drop down to writing raw SQL
queries but was trying to see if there was a way I could do something like
this (if this is actually a smart query to do at all) via the ORM. I'm
currently looking at the extra() command but can't seem to get the right
mojo to get that working.
What's the right way to get this one working?
Thanks
-J
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