_QuerySet.__nonzero__ returns True for empty query sets with cached results

[Brodie Rao]

Given any sort of model with zero instances/rows, the following will currently occur:

>>> objects = Foo.objects.all()
>>> bool(objects)
False
>>> bool(objects)
True

_QuerySet.__nonzero__ unconditionally returns True when the result cache is populated, even if nothing is in the cache (i.e. there are no results).

I'm attaching a patch that fixes this behavior, and a test case for it.

[Brodie Rao]

Fixes _QuerySet.nonzero for empty query sets (with test case)

[cide@…]

While this patch fixes the nonzero issue, it appears to break stuff as well. More specifically, while running this patch, django attempted to insert several already existing entries into the database for some reason (with the same primary key) - resulting in several IntegrityErrors. Removing the patch immediately fixed the issue.

[cide@…]

Fixes _QuerySet.nonzero for empty query sets (with test case). Works with ValuesQuerySets.

[cide@…]

I have attached a patch that fixes the issues I had. The first patch relied on QuerySet._result_cache evaluating to True in a boolean context after iter(instance).next() was called. This was not the case with ValuesQuerySets. In this patch, nonzero returns True if the next() call does not raise StopIteration.

[Malcolm Tredinnick]

(In [7417]) Fixed #6899 -- Fixed a problem with boolean evaluation of empty querysets.
Based on patches from cide@… and brodie.