Matt,
I know I can do this, but if I want to write a generic view that
receives a queryset and deletes everything that isn't in this
queryset? I'm not having this problem right now. I just thought that
this was a feature that would be interesting to have in the ORM. And I
am asking if it already exists to think of suggesting it to be
integrated to the framework. I think the queryset should keep track of
it self. It knows what is the filter, so why can't it negate this
filter?
On Mar 23, 10:40 am, Paulo Almeida <igcbioinformat...@gmail.com>
wrote:
> Is there some way you can keep track of the queryset? I understand it's
> dynamic, and maybe complex, for you to be looking for this kind of solution,
> but if it were possible to have a variable or dictionary keeping track of
> what you add to the queryset, it might be possible to use it to build the
> inverse.
>
> - Paulo
>
> On Tue, Mar 23, 2010 at 1:23 PM, Vinicius Mendes | meiocodigo.com <
>
>
>
> vbmen...@gmail.com> wrote:
> > Ok. The code proposed by Tim Shaffer works and gives only one query.
> > But it makes use of subselects, what is heavy for the database. Take a
> > look at the generated SQL:
>
> > 'SELECT `auth_user`.`id`, `auth_user`.`username`,
> > `auth_user`.`first_name`, `auth_user`.`last_name`,
> > `auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`,
> > `auth_user`.`is_active`, `auth_user`.`is_superuser`,
> > `auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user`
> > WHERE NOT (`auth_user`.`id` IN (SELECT U0.`id` FROM `auth_user` U0
> > WHERE U0.`first_name` = vinicius )) LIMIT 21'
>
> > I was thinking of something like:
>
> > 'SELECT `auth_user`.`id`, `auth_user`.`username`,
> > `auth_user`.`first_name`, `auth_user`.`last_name`,
> > `auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`,
> > `auth_user`.`is_active`, `auth_user`.`is_superuser`,
> > `auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user`
> > WHERE NOT `auth_user`.`first_name` = vinicius LIMIT 21'
>
> > Why hit the database if I can do this with boolean login? Why not
> > negate the filter condition instead of doing a subselect?
>
> > On Mar 22, 7:15 pm, Tim Shaffer <timster...@gmail.com> wrote:
> > > It depends. This will only run one query, when negated_queryset is
> > > used.
>
> > > queryset = User.objects.filter(first_name='vinicius')
> > > negated_queryset = User.objects.exclude(id__in=queryset.values("id"))
>
> > > Since the first queryset is not evaluated until it's used in the
> > > negated_queryset (as a subquery).
>
> > > On Mar 22, 5:55 pm, Matt Schinckel <m...@schinckel.net> wrote:
>
> > > > On Mar 23, 6:17 am, Phlip <phlip2...@gmail.com> wrote:
>
> > > > > > Just create another queryset that excludes everything in your first
> > > > > > queryset:
>
> > > > > > negated_queryset =
> > User.objects.exclude(id__in=queryset.values("id"))
>
> > > > > QuerySets are already so easy to plug-n-play... Ain't there a way to
> > > > > do it without whacking the database twice?
>
> > > > Are you sure it hits the db twice? I seem to recall a similar case
> > > > where I thought I was, but since the queryset evaluation is lazy, the
> > > > ORM potentially has the ability to make this into a single query.
>
> > > > (I can't recall if when I did this sort of thing I was using
> > > > SQLAlchemy, but I _think_ it was pure django).
>
> > > > Matt.
>
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