#23662: QuerySet __nonzero__, __len__ cause queryset evaluation
-------------------------------------+-------------------------------------
Reporter: smishlayev | Owner: nobody
Type: | Status: new
Cleanup/optimization | Version: 1.7
Component: Database layer | Resolution:
(models, ORM) | Triage Stage:
Severity: Normal | Unreviewed
Keywords: | Needs documentation: 0
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 1
Easy pickings: 0 |
-------------------------------------+-------------------------------------
Old description:
> Current implementation:
> https://github.com/django/django/blob/master/django/db/models/query.py
> class QuerySet(object):
> ...
> def __nonzero__(self):
> self._fetch_all()
> return bool(self._result_cache)
> ...
> def __len__(self):
> self._fetch_all()
> return len(self._result_cache)
>
> These methods call self._fetch_all(), thus evaluating the queryset.
> Although, this behaviour is documented
> (https://docs.djangoproject.com/en/1.7/ref/models/querysets/#when-
> querysets-are-evaluated), it is not obvious.
>
> It seems logical to evaluate queryset, when casting it to a list() or
> iterating over it, but these particular cases have nothing to do with the
> queryset contents. There exist specific lazy methods (QuerySet.exists()
> and QuerySet.count() respectively) which, IMHO, should be used for the
> magic method implementation.
>
> If I already have fetched the results of a queryset, I'm okay to call
> __len__() on them, but if they haven't been retrieved yet, I'd rather use
> SQL COUNT() instead. That is exactly, what QuerySet.count() does. The
> same goes for the __nonzero__() and exists().
New description:
Current implementation:
https://github.com/django/django/blob/master/django/db/models/query.py
{{{
class QuerySet(object):
...
def __nonzero__(self):
self._fetch_all()
return bool(self._result_cache)
...
def __len__(self):
self._fetch_all()
return len(self._result_cache)
}}}
These methods call self._fetch_all(), thus evaluating the queryset.
Although, this behaviour is documented
(https://docs.djangoproject.com/en/1.7/ref/models/querysets/#when-
querysets-are-evaluated), it is not obvious.
It seems logical to evaluate queryset, when casting it to a list() or
iterating over it, but these particular cases have nothing to do with the
queryset contents. There exist specific lazy methods (QuerySet.exists()
and QuerySet.count() respectively) which, IMHO, should be used for the
magic method implementation.
If I already have fetched the results of a queryset, I'm okay to call
__len__() on them, but if they haven't been retrieved yet, I'd rather use
SQL COUNT() instead. That is exactly, what QuerySet.count() does. The same
goes for the __nonzero__() and exists().
--
Comment (by dfunckt):
Edited bug description to properly format example code
--
Ticket URL: <https://code.djangoproject.com/ticket/23662#comment:2>
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