Django Q object not chaining correctly

Question

I'm querying a ManyToMany field (tags). The values come from a list:

tag_q = Q()

tag_list = ["work", "friends"]
for tag in tag_list:
    tag_q &= Q(tags__tag=tag)

Post.objects.filter(tag_q)

When I have only one value, it works flawlessly, but when more objects are stacked it always return an empty queryset.

I didn't used tags__tag__in=tag_list because it returned any post that contained any of the tags in the tag list (an OR filter), and I need an AND filter here.

This is my models:

class Tag(models.Model):
    tag = models.CharField(max_length=19, choices=TagChoices.choices())

class Post(models.Model):
    tags = models.ManyToManyField(Tag, related_name='posts', blank=True)

This is the Q object that is being passed in the filter query:

(AND: ('tags__tag', 'work'), ('tags__tag', 'friends')

You can not work with a Q object like that: a filter is an existential quantifier, not a universal. It means you are looking for a single Tag that has as tag name work and friends at the same time. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Oct 29, 2021 at 10:27

willeM_ Van Onsem · Accepted Answer · 2021-10-29 10:30:09Z

1

You can not work with a Q object like that: a filter is an existential quantifier, not a universal. It means you are looking for a single Tag that has as tag name work and friends at the same time.

What you can do is work with a list of tags, and then count if the number of Tags is the same as the number of items to search for, like:

tag_list = ['work', 'friends']

Post.objects.filter(tags__tag__in=tag_list).annotate(
    ntags=Count('tags')
).filter(ntags=len(set(tag_list))

since django-3.2, you can work with .alias(…) [Django-doc] instead of .annotate(…) [Django-doc]:

tag_list = ['work', 'friends']

Post.objects.filter(tags__tag__in=tag_list).alias(
    ntags=Count('tags')
).filter(ntags=len(set(tag_list))

answered Oct 29, 2021 at 10:30

willeM_ Van Onsem

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