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?
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
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.
您可以编写一个返回相应日期的方法(如果汽车有所有者返回生日,则返回creation_date),然后根据此方法订购模型。
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