[Django]-Django queryset optimisation: Reverse lookup on _set with filter

Try to use annotate with conditional expressions. Your query will look like this:

from django.db.models import Q, F, Sum, Case, When, IntegerField
from django.db.models.functions import Coalesce


cartlist = Cart.objects.annotate(
    sum=Coalesce(Sum(Case(
        When(Q(carttax__tax__type__name=taxtype) & Q(carttax__tax__tax=tax), then=F('carttax__tax_amount')),
        output_field=IntegerField()
        )), 0)
    )

And in template:

{% for cart in cartlist %}
{{ cart.sum }}
{% endfor %}