django annotate and count: how to filter the ones to include in count

Question

Given a queryset, I add the count of related objects (ModelA) with the following:

qs = User.objets.all()
qs.annotate(modela__count=models.Count(\'modela\'))
         


        

Answer 1

In a LEFT JOIN, every field of modela could be NULL because of the absence of corresponding row. So

modela.deleted_at IS NULL

...is not only true for the matching rows, but also true for those users whose have no corresponding modela rows.

I think the right SQL should be:

SELECT COUNT(
    CASE
      WHEN
        `modela`.`user_id` IS NOT NULL  -- Make sure modela rows exist
        AND `modela`.`deleted_at` IS NULL
        THEN 1
      ELSE NULL
    END
  ) AS `modela__count`,
  `users`.*
FROM `users`
LEFT OUTER JOIN `modela`
  ON ( `users`.`id` = `modela`.`user_id` )
GROUP BY `users`.`id`

In Django 1.8 this should be:

from django.db import models
qs = User.objects.all().annotate(
    modela_count=models.Count(
        models.Case(
            models.When(
                modela__user_id__isnull=False,
                modela__deleted_at__isnull=True,
                then=1,
            )
        )
    )
)

Notice:

@YAmikep discovered that a bug in Django 1.8.0 makes the generated SQL have an INNER JOIN instead of a LEFT JOIN, so you will lose rows without corresponding foreign key relationship. Use Django 1.8.2 or above version to fix that.