Django custom order_by

Question

I've got a model eg. Car with a foreign key eg. Owner, which may or may not be blank. The Car has a creation_date.

I would like to order these cars by date, but if the car has an owner, the date of birth of the owner must be taken instead of the creation_date of the car.

Is this possible?

Answer 1

Have a look at this similar question: Good ways to sort a queryset? - Django

You can't use the model's Meta ordering as it only accepts one field

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

You can't use the query order_by('creation_date', 'birthdate') as it only sorts by birthdate if they have the same creation_date

So, you could write a custom manager to do incorporate a custom sort for you.

import operator
class CarManager(models.Manager):
    def get_query_set(self):
        auths = super(CarManager, self).get_query_set().all().order_by('-creation')
        ordered = sorted(auths, key=operator.attrgetter('birthday'))
        return ordered

class Car(models.Model):
    sorted = CarManager()

so now you can query:

Car.sorted.all()

to get all a queryset of sorted car's

Answer 2

This is possible by falling back to SQL:

Car.objects.filter(...).extra(select={'odate': '''
  if(owner_id,
     (select date_of_birth from owner_table where id=owner_id),
     creation_date
  )
'''}).order_by('odate')

if function is MySQL-specific. In case of SQLite or Postgres you should use case statement.

Answer 3

You could write a method that returns the appropriate date (if car has owner return birthday else return creation_date) and then order your model based on this method.