#26426: Add a way to customize a QuerySet's joins
-------------------------------------+-------------------------------------
Reporter: yourcelf | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: 1.9
(models, ORM) |
Severity: Normal | Resolution:
Keywords: QuerySet.extra | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by timgraham):
* stage: Unreviewed => Accepted
Old description:
> This ticket is just to document a use case for QuerySet.extra as
> requested by the docs:
> https://docs.djangoproject.com/en/1.9/ref/models/querysets/#extra
>
> I have a Category model like this:
>
> {{{
> class Category(models.Model):
> name = models.CharField(max_length=200)
> followers = models.ManyToManyField(User)
> }}}
>
> I want to get a list of all categories, but to annotate each category
> with whether the currently logged in user is a "follower" of the
> category. Neither `prefetch_related` nor `annotate` work here, because I
> don't want to fetch nor aggregate over //all// "followers" (potentially
> many), I just want the presence of the current user. The extra query
> looks like this:
>
> {{{
> Category.objects.filter(...).extra(
> select={'is_following': '''EXISTS(
> SELECT "id" FROM "projects_category_followers" WHERE
> "projects_category_followers"."category_id"="projects_category"."id" AND
> "projects_category_followers"."user_id"=%s
> )'''},
> select_params=(request.user.id,)
> )
> }}}
New description:
This ticket is just to document a use case for QuerySet.extra as requested
by the docs:
https://docs.djangoproject.com/en/1.9/ref/models/querysets/#extra
I have a Category model like this:
{{{
class Category(models.Model):
name = models.CharField(max_length=200)
followers = models.ManyToManyField(User)
}}}
I want to get a list of all categories, but to annotate each category with
whether the currently logged in user is a "follower" of the category.
Neither `prefetch_related` nor `annotate` work here, because I don't want
to fetch nor aggregate over //all// "followers" (potentially many), I just
want the presence of the current user. The extra query looks like this:
{{{
Category.objects.filter(...).extra(
select={'is_following': '''EXISTS(
SELECT "id" FROM "projects_category_followers" WHERE
"projects_category_followers"."category_id"="projects_category"."id" AND
"projects_category_followers"."user_id"=%s
)'''},
select_params=(request.user.id,)
)
}}}
--
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Ticket URL: <https://code.djangoproject.com/ticket/26426#comment:4>
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