Django: Adding “NULLS LAST” to query

Question

I would like to sort a model by using Postgresql\'s \"NULLS LAST\" option. How could it be done?

I tried something like

MyModel.objects.all().extra(ord

Answer 1

There is an another way to add managed nulls functionality to Django < v1.11 with Django v1.11 style:

from my_project.utils.django import F
MyModel.objects.all().order_by(F('price').desc(nulls_last=True))
# or
MyModel.objects.all().order_by(F('price').desc().nullslast())

Cons:

1. Easy migration to Django 1.11
2. We don't get deep into query compiler internals

To do so we need to override django.db.models.F and django.db.models.expressions.OrderBy classes:

from django.db.models import F as DjangoF
from django.db.models.expression import OrderBy as DjangoOrderBy


class OrderBy(DjangoOrderBy):
    def __init__(self, expression, descending=False, nulls_last=None):
        super(OrderBy, self).__init__(expression, descending)
        self.nulls_last = nulls_last
    ...

    def as_sql(self, compiler, connection, template=None, **extra_context):
        ...
        ordering_value = 'DESC' if self.descending else 'ASC'
        if self.nulls_last is not None:
            nulls_value = 'LAST' if self.nulls_last else 'FIRST'
            ordering_value += ' NULLS ' + nulls_value

        placeholders = {
            'expression': expression_sql,
            'ordering': ordering_value,
        }
        ...

    def nullslast(self):
        self.nulls_last = True

    def nullsfirst(self):
        self.nulls_last = False


class F(DjangoF):
    ...

    def asc(self, nulls_last=None):
        return OrderBy(self, nulls_last=nulls_last)

    def desc(self, nulls_last=None):
        return OrderBy(self, descending=True, nulls_last=nulls_last)

Answer 2

This was probably not available when the question was asked, but since Django 1.8 I think this is the best solution:

from django.db.models import Coalesce, Value
MyModel.objects.all().annotate(price_null=
    Coalesce('price', Value(-100000000)).order_by('-price_null')

Coalesce selects the first non-null value, so you create a value price_null to order by which is just price but with null replaced by -100000000 (or +?).