How can I get Q objects from a filter set?

[taobojlen]

I'm using django-filter to filter down querysets.

Now, in another part of my code, I want to use the same filter set, but I won't want to filter a queryset -- instead, I want to filter an aggregation. Is is possible to get a Q-expression from django-filter?

For example, say I have

class Bar(models.Model):
  foo = models.ForeignKey(Foo, on_delete=models.CASCADE)

class Foo(models.Model):
  def get_bar_count(self, filters: dict[str, Any]) -> int:
    bar_filter = BarFilter(data=filters, queryset=self.bar_set.all())
    return bar_filter.qs.count()

and now I want to use this as an annotation. Can I do something along the lines of

bar_filter = BarFilter(data=filters, queryset=Bar.objects.all())
foos = Foo.objects.all().annotate(bar_count=Count("bar_set", filter=bar_filter.get_q_objects()))

[KianAnbarestani]

Use the .qs property to get the filtered queryset, then extract the .query.where if needed — but for annotations, a cleaner way is to build your own Q manually from the filter form:

from django.db.models import Q

def extract_q_from_filter(filter_instance):
    q = Q()
    for name, value in filter_instance.form.cleaned_data.items():
        if value not in (None, '', []):
            q &= Q(**{name: value})
    return q

Then:

bar_filter = BarFilter(data=filters, queryset=Bar.objects.all())
if bar_filter.is_valid():
    bar_q = extract_q_from_filter(bar_filter)
    foos = Foo.objects.annotate(
        bar_count=Count('bar_set', filter=bar_q)
    )

NOTE:

1. This assumes simple filters (i.e., field=value). If you're using custom method=... filters or complex lookups, you'll need to extend the logic accordingly.
2. This is a workaround; Django Filter is not designed to expose Q objects directly.

Let me know if you want a reusable utility that supports nested field names or __in lookups.