Use case for QuerySet.extra: annotate query with the existence of a relation

[Charlie DeTar]

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,)
)