annotating across a ManyToManyField returns duplicate results

[Seti]

Dublication objects, if using annotation with models.F() (not the models.Count - it is working properly) on ManyToManyField

Code:

print('Real count books =\t', Book.objects.count())
print('Total count tags in all books =\t', 
    Book.objects.annotate(
        count_tags=models.Count('tags', distinct=True)
    ).aggregate(
            total_used_count_tags=models.Sum('count_tags')
    )['total_used_count_tags']
)

qs = Book.objects.values('tags').annotate(tags_names=models.F('tags__name'))
print('Query:\n', qs.query)
print('Count books after query =\t', qs.count())


print('Raw query return next count books =\t', len([i for i in Book.objects.raw("""
    SELECT
        "books"."id",
        "books_tags"."tag_id",
        "tags"."name" AS "tags_names"
    FROM "books"
    LEFT OUTER JOIN
        "books_tags" ON ("books"."id" = "books_tags"."book_id")
    LEFT OUTER JOIN
        "tags" ON ("books_tags"."tag_id" = "tags"."id")
    ORDER BY
        "books"."date_added" ASC
    """
)]))

Output

Real count books = 5
Total count tags in all books = 13
Query: SELECT "books_tags"."tag_id", "tags"."name" AS "tags_names" FROM "books" LEFT OUTER JOIN "books_tags" ON ("books"."id" = "books_tags"."book_id") LEFT OUTER JOIN "tags" ON ("books_tags"."tag_id" = "tags"."id") ORDER BY "books"."date_added" ASC
Count books after query = 13
Raw query return next count books = 13

Models:

class Book(models.Model):
    """
    Model for books
    """

    id = models.UUIDField(primary_key=True, editable=False, default=uuid.uuid4)
    #other fields
    tags = models.ManyToManyField(
        Tag,
        verbose_name=_('Tags'),
        related_name='books',
    )


class Tag(models.Model):
    """
    Model tags for another models
    """

    name = models.SlugField(
        _('name'),
        max_length=30,
        unique=True,
        allow_unicode=True,
    )

[Shai Berger]

Hi,

We value your interest in the Django project. However, you are asking questions in the wrong place and in the wrong way.

Please don't open an issue unless you are fairly convinced that you found a problem with Django.

With respect to how to ask questions -- it is not about the English language, it is about the content of the tickets you open. Please see e.g. ​How to ask smart questions -- the link points directly to the list of translations, so you can hopefully read it in your language.

For the specific matter you raised here -- yes, I believe this is the result one should expect. It is not a bug. For what you are trying to achieve, try to use prefetch_related.

[Tim Graham]

#10060 might be relevant.

[Seti]

Replying to shaib:

Hi,

We value your interest in the Django project. However, you are asking questions in the wrong place and in the wrong way.

Please don't open an issue unless you are fairly convinced that you found a problem with Django.

With respect to how to ask questions -- it is not about the English language, it is about the content of the tickets you open. Please see e.g. ​How to ask smart questions -- the link points directly to the list of translations, so you can hopefully read it in your language.

For the specific matter you raised here -- yes, I believe this is the result one should expect. It is not a bug. For what you are trying to achieve, try to use prefetch_related.

Please, delete this ticket