queryset.order_by().first() is not consistent with other queryset behaviours

[Iuri de Silvio]

For any other operation, an empty .order_by() remove default ordering, but it orders by pk when .first() is called.

>>> User.objects.only("pk").order_by()[:1]
SELECT "auth_user"."id"
  FROM "auth_user"
LIMIT 1

>>> User.objects.only("pk").order_by().first()
SELECT "auth_user"."id"
  FROM "auth_user"
ORDER BY "auth_user"."id" ASC
LIMIT 1

I think it is an "undefined behavior" and it should build the query without ordering.

The ~almost~ undocumented implementation for this today should be:

try:
    user = User.objects.only("pk")[0]
except IndexError:
    user = None

[Mariusz Felisiak]

We added order_by("pk") only when it's not explicitly ordered, mainly, to get the consistent and deterministic behavior on all backends. It's been that way basically forever and, IMO, we shouldn't change it. You can start a discussion on DevelopersMailingList if you don't agree.

[Iuri de Silvio]

Thanks! I disagree a bit because if I'm explicitly disabling ordering with an empty order_by, it is not the same as not defining ordering.

I'll follow up on mailing list.

[Mariusz Felisiak]

See related ticket #23805.

[David Sanders]

Thanks! I disagree a bit because if I'm explicitly disabling ordering with an empty order_by, it is not the same as not defining ordering.

I get your original point, but I agree with Mariusz and the original authors adding this behaviour.

To have a first/last you must have deterministic ordering and not whatever the database feels like it wants to do otherwise this will start to introduce subtle soul-destroying bugs. I've numerous tests fail intermittently (with the same db) because we forgot to add explicit ordering.