Do using queryset.count() caches the queryset?

December 9, 2021

I’m making some filtering in my endpoint, and one of the filters are only applied if the filtered queryset has more than 30 items.

yesterday_date = timezone.now() - timezone.timedelta(days=1)
if query_dict.get("active"):
    active_query = cleaned_query.filter(created_at__gt=yesterday_date)
    if active_query.count() > 30:
        cleaned_query = active_query
    else:
        cleaned_query = cleaned_query[:30]

My doubt is, will the .count() method already evaluates and caches the queryset or should I use len(queryset) to avoid another database hit in case it’s bigger than 30?

>Solution :

If you check django’s docs about When Querysets Are Evaluated

You’ll see some information about counting…

Note: If you only need to determine the number of records in the set
(and don’t need the actual objects), it’s much more efficient to
handle a count at the database level using SQL’s SELECT COUNT(*).
Django provides a count() method for precisely this reason.

So a count() doesn’t evaluate the set, whereas a len() does.