Django ORM apply .filter() to an existing queryset

I'm a bit confused about requests into database to be executed in this code snippet

>>> from core.models import People
>>> p = People.objects.all()
>>> p.filter(age__gt=10) 

It's clear that passing objects.all() into ORM will lead to one request like SELECT *

However, if I apply .filter() method to an existing queryset, will it lead to second request to database? Or ORM will look through the existing queryset?

Thank you

Solution 1:

However, if I apply .filter() method to an existing queryset, will it lead to second request to database?

constructing querysets will not make a database query. Indeed, here p = People.objects.all() will not make a query.

Querysets will make a query if you "consume" the queryset. You consume the queryset if you iterate over it, call len(…), str(…) en repr(…) over it, etc.

This thus means that if you print p.filter(age__gt=10), it will only make a query with SELECT people.* FROM people WHERE age > 10. If you later for example print p, it will make a second query SELECT people.* FROM people.

Querysets also cache the result when they are consumed. So if you iterate a second time on the same QuerySet, it will reuse the result of the previous query.