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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