Add __nonzero__ to allow querysets to efficiently test for emptiness

[Michael Axiak <axiak@…>]

Currently, if you do:

if Model.objects.filter(Complicated):
   ...

you're asking for performance trouble, since it will issue a __len__ statement on it. Most people will solve this problem by doing:

if Model.objects.filter(Complicated).count() > 0:
   ...

But this is still much slower than doing a SELECT LIMIT 1. Thus, I propose adding a __nonzero__ function, so that the first method of checking for emptiness does the right thing. (​http://docs.python.org/ref/customization.html)

[Michael Axiak <axiak@…>]

Simple patch to add __nonzero__ to queryset

[Malcolm Tredinnick]

Dupe of #2430.

Coincidentally, I started working on this problem (and a couple of other len()-related problems) last night and I've got a fix that doesn't require the extra query, using the technique at the end of that ticket.

So, thanks for the patch, but we can fix it in a way that incurs zero extra queries. Should have something in later today or tomorrow, depending on how much the universe hates me.