Re: QuerySet: "if obj in queryset" can be very slow

On Wed, Nov 2, 2011 at 6:26 PM, Ian Clelland <clelland@gmail.com> wrote:
> Just looking at the source to QuerySet (finally), and it looks like the
> __contains__ method actually does something different than this: It
> evaluates the whole QuerySet in bulk at the database level, and starts
> creating model instances based on that, but only until it finds a matching
> one. So, after running "if obj in qs", you might end up with one object
> created, or you might end up with 70M objects, or anywhere in between.
> Again: odd, undocumented, and potentially surprising behaviour, and I'd
> recommend explicit over implicit, especially in this case.
>

I disagree. All of this behaviour is documented in my mind:

It is documented here when querysets are evaluated:

https://docs.djangoproject.com/en/1.3/ref/models/querysets/#when-querysets-are-evaluated

It's documented here that iterating through a queryset will cache the
objects created:

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

It's documented here that testing for results is slightly quicker than
evaluating and testing the queryset:

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

It's documented here that caching is optimistic - it caches the
results that have been requested:

https://docs.djangoproject.com/en/1.3/topics/db/queries/#caching-and-querysets

Do we really need to specifically point out that the __contains__
method iterates through the queryset and stops when it finds an
answer?

Cheers

Tom

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