Trap in counting related objects in Django

Trap in counting related objects in Django

Task: for every object count number of related objects satisfying some conditions.

Example:

class Category(models.Model):
    title = models.CharField(max_length=50)

class Article(models.Model):
    title = models.CharField(max_length=50)
    category = models.ForeignKey(Category)
    approved_at = models.DateTimeField(blank=True, null=True)

Pay attention at field Article.approved_at, it contains article approval time and it can be null.

Create test data:

from django.utils import timezone

c1 = Category.objects.create(title='c1')
c2 = Category.objects.create(title='c2')

a1 = Article.objects.create(category=c1, title='a1')
a2 = Article.objects.create(category=c1, title='a2', approved_at=timezone.now())

So we have two categories. The first one has one approved and one not approved article. The second category doesn't have any article.

Let's begin with counting all articles for each category:

from django.db.models import Count

>>> Category.objects.annotate(
...     article_count=Count('article')
... ).values('title', 'article_count')

<QuerySet [{'article_count': 2, 'title': u'c1'}, {'article_count': 0, 'title': u'c2'}]>

Django ORM built following expected SQL query:

SELECT "main_category"."title", COUNT("main_article"."id") AS "article_count"
    FROM "main_category"
    LEFT OUTER JOIN "main_article" ON ("main_category"."id" = "main_article"."category_id")
    GROUP BY "main_category"."id", "main_category"."title";

Ok, now we want to count only approved articles. Using SQL language, we can just update JOIN part:

SELECT "main_category"."title", COUNT("main_article"."id") AS "article_count"
    FROM "main_category"
    LEFT OUTER JOIN "main_article" 
        ON ("main_category"."id" = "main_article"."category_id" AND
            "main_article"."approved_at" IS NOT NULL)
    GROUP BY "main_category"."id", "main_category"."title";

Unfortunately, Django ORM doesn't allow to apply any filter for Count (at least in v1.10). But starting from v1.8 we have conditional expressions, so let's use it:

from django.db.models import Count, Case, When

>>> Category.objects.annotate(
...     article_count=Count(
...         Case(When(article__approved_at__isnull=False, then=1))
...     )
... ).values('title', 'article_count')

<QuerySet [{'article_count': 1, 'title': u'c1'}, {'article_count': 0, 'title': u'c2'}]>

The returned values are valid. Corresponding SQL query:

SELECT "main_category"."title", COUNT(
    CASE WHEN "main_article"."approved_at" IS NOT NULL THEN 1 ELSE NULL END
) AS "article_count"
FROM "main_category"
LEFT OUTER JOIN "main_article" ON ("main_category"."id" = "main_article"."category_id")
GROUP BY "main_category"."id", "main_category"."title";

The Trap

Here is an interesting question, how we can count number of not approved articles?

The first thought that comes to the mind is to change False to True in the query:

>>> Category.objects.annotate(
...     article_count=Count(
...         Case(When(article__approved_at__isnull=True, then=1))
...     )
... ).values('title', 'article_count')

But query returns not valid values:

<QuerySet [{'article_count': 1, 'title': u'c1'}, {'article_count': 1, 'title': u'c2'}]>

The second category has mysterious unapproved article.

Check the SQL:

SELECT "main_category"."title", COUNT(
    CASE WHEN "main_article"."approved_at" IS NULL THEN 1 ELSE NULL END
) AS "article_count"
FROM "main_category"
LEFT OUTER JOIN "main_article" ON ("main_category"."id" = "main_article"."category_id")
GROUP BY "main_category"."id", "main_category"."title";

The condition

CASE WHEN "main_article"."approved_at" IS NULL THEN 1

will be true even if category doesn't have any article at all!

One approach to fix is the following:

>>> Category.objects.annotate(
...     article_count=Count(
...         Case(
...             When(
...                 article__id__isnull=False,
...                 article__approved_at__isnull=True,
...                 then=1
...             )
...         )
...     )
... ).values('title', 'article_count')

<QuerySet [{'article_count': 1, 'title': u'c1'}, {'article_count': 0, 'title': u'c2'}]>

The moral of the story

When condition like IS NULL happens, double check every edge case!

Published: Jan. 14, 2017
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