Ignore null values in descending order using Django Rest Framework

Question

I am using Django for my website, and hence decided to use Django Rest Framework for building my REST APIs. For a particular model, i want to filter on a text field (using SearchFilter for that), filter on a few categorical fields (FilterBackend with a FilterSet defined) and be able to order data based on some fields (OrderingFilter for this).

class StatsAPI(generics.ListAPIView):
    model = Stats
    queryset = Stats.objects.all()
    serializer_class = StatsSerializer
    filter_backends = (filters.DjangoFilterBackend, filters.OrderingFilter, filters.SearchFilter)
    filter_class = StatsFilter
    pagination_class = StatsPagination
    ordering_fields = ('__all__')
    search_fields = ('display_name')

The issue i am facing is with my ordering fields as they also contain nulls. Ordering in ascending order works fine. However ordering in descending order (www.example.com/api/stats/?ordering=-appearance), pushes the null values to the top.

How do i ignore the null values when using descending order? The number of fields on which ordering can be performed are roughly 20 in number.

Answer 1

This is a slightly different solution -- rather than filtering null out, this replacement for filters.OrderingFilter just always makes sure they sort last:

class NullsAlwaysLastOrderingFilter(filters.OrderingFilter):
    """ Use Django 1.11 nulls_last feature to force nulls to bottom in all orderings. """
    def filter_queryset(self, request, queryset, view):
        ordering = self.get_ordering(request, queryset, view)

        if ordering:
            f_ordering = []
            for o in ordering:
                if not o:
                    continue
                if o[0] == '-':
                    f_ordering.append(F(o[1:]).desc(nulls_last=True))
                else:
                    f_ordering.append(F(o).asc(nulls_last=True))

            return queryset.order_by(*f_ordering)

        return queryset

Answer 2

You can custom your own OrderingFilter:

# Created by [email protected] at 2022/8/13
from django.db.models import F, OrderBy
from django_filters import rest_framework as filters


class MyOrderingFilter(filters.OrderingFilter):
    def get_ordering_value(self, param):
        value = super().get_ordering_value(param)
        return OrderBy(F(value.lstrip("-")), descending=value.startswith("-"), nulls_last=True)