id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26426	Add a way to customize a QuerySet's joins	Charlie DeTar	nobody	"

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 feature	new	Database layer (models, ORM)	1.9	Normal		QuerySet.extra		Accepted	0	0	0	0	0	0