Django models order_by non db field

Question

I have this model:

class Foo(models.Model):
    foo_id = models.IntegerField()
    foo_old_prize = models.FloatField()
    foo_new_prize = models.FloatField()

def get_dif(self):
    return self.foo_old_prize  - self.foo_new_prize

Is there some trick how to do this Foo.objects.all().order_by('get_dif') ?

Thx

Answer 1

Look into the extra Django queryset operator:

q = Foo.objects.extra(select={'dif': 'foo_old_prize - foo_new_prize'})
q = q.extra(order_by = ['dif'])

Answer 2

You can use extra for this:

Foo.objects.extra(
    select={'diff':'foo_old_prize - foo_new_prize'},
    order_by=('diff',)
)