Boolean test on queryset

[Adam Nelson]

When one tests for the boolean value of a queryset in the following way:

examples = Example.objects.all()

if examples:
    ..... do something


You might think (I did) that Django will call a __nonzero__ special attribute that would either execute an EXISTS SQL STATEMENT or a SELECT statement with a LIMIT of 1.  Instead, the ORM evaluates the entire queryset through the __len__ attribute.

Is there a best practice for working around this?

Thanks,

Adam

[Jirka Vejrazka]

Hi Adam,

I tend to use:

if examples.count():

...something...

HTH

Jirka

[Adam Nelson]

Jirka,

That doesn't solve the problem.  That will still do a very expensive count() operation on the queryset.  In fact, examples.count() is what happens when you do bool(examples) anyway.

Thanks,

Adam

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[Ian Clelland]

On Monday, November 28, 2011, Adam Nelson <ad...@yipit.com> wrote:
> Jirka,
> That doesn't solve the problem.  That will still do a very expensive count() operation on the queryset.  In fact, examples.count() is what happens when you do bool(examples) anyway.

I'm confused here -- examples.count() is definitely not the same as examples.__len__(), which is what you originally claimed that bool() was doing. examples.count() should issue a SELECT COUNT query, while len() iterates through the rows in the result set.

If you just need to know whether at least one row exists in the result, use examples.exists() -- that's what it's for.

https://docs.djangoproject.com/en/1.3/ref/models/querysets/#exists

From the (linked) docs:

> Returns True if the QuerySet contains any results, and False if not. This tries to perform the query in the simplest and fastest way possible, but it does execute nearly the same query. This means that calling QuerySet.exists() is faster than bool(some_query_set), but not by a large degree.

> Thanks,
> Adam
>
> On Mon, Nov 28, 2011 at 8:11 PM, Jirka Vejrazka <jirka.v...@gmail.com> wrote:
>>
>> Hi Adam,
>>
>>  I tend to use:
>>
>>  if examples.count():
>>
>>    ...something...
>>
>>  HTH
>>
>>    Jirka
>>
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>>
>
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>

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Regards,
Ian Clelland
<clel...@gmail.com>