Django admin: order by a nullable date, with null values at the end

I have a model with a DateTimeField which I would like to use to order the table in admin. Using ordering = ('-completed',) in admin.py works, but those with null values come first in the sort. Is there a way to make them appear at the end instead?

So at the moment I have something like:

Column A     Completed       Column C
x            -               y
x            -               y
x            Today's date    y
x            Yesterday's     y

Is there a way to make it like this?

Column A     Completed       Column C
x            Today's date    y
x            Yesterday's.    y
x            -               y
x            -               y

You can use query expressions. To order by completed ascending and make null values sort last, use this:

from django.db.models import F

ordering = [F('completed').desc(nulls_last=True)]

Source :https://docs.djangoproject.com/en/dev/ref/models/options