如何正确过滤掉反向的ManyToMany？

Question

I can't figure out how to filter reversed ManyToMany in Django . I want to get LanguageLevel objects, connected with at least one UserProfile .

In human: All languages such as at least one User can speak this Language.

---

I have a model:

class LanguageLevel(models.Model):
    ...

and

class UserProfile(models.Model):
    ...
   language_levels = models.ManyToManyField('LanguageLevel', blank=True, related_name='translators')
    ...

Already tried:

active_language_levels = LanguageLevel.objects.filter(lambda x: len(x.translators)!=0)

AND

for language_level in LanguageLevel.objects.all():
        print language_level.translators
        if len(language_level.translators)>0:
            print 'ok'
            print language_level
        else:
            print 'no'
            print language_level

Answer 1

I think you are looking for something like this:

from django.db.models import Count

LanguageLevel.objects.all() \
                .annotate(translators_count=Count('translators__id')) \
                .filter(translators_count__gte=1)

Just add proper link on translators__id to count distinct entries of each user.

If you want to select languages for specific user, just pass proper queryset before annotate:

LanguageLevel.objects.filter(translators__id=user_id)
                .annotate(...)

Answer 2

You cannot filter by a M2M field using django orm. Instead you can follow this workaround,

LanguageLevel.objects.all().values('language_levels__id')