id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
30667	Running last() on a related object with a F() ordering mutates model ordering.	Mikail	nobody	"Hi,

I have ran into a weird issue when working with a O2M relation, where one would use the meta field ordering with `F(...)`.

We can take this following code to illustrate as an example:

{{{#!python
from django.db import models
from django.db.models import F


class Item(models.Model):
    pass


class ItemValue(models.Model):
    sort_order = models.IntegerField(unique=True, null=True)
    item = models.ForeignKey(Item, on_delete=models.CASCADE, related_name=""items"")

    class Meta:
        ordering = (F(""sort_order"").asc(nulls_last=True), ""id"")
}}}


In `Item` we have a relation to the `items`. The most important part here, is the ordering with a `F(...)`.

Now, if another part of the code (a view, a test, ...) runs `last()` on the items, it will trigger some kind of switch in the ORM, which will make it always run `F(...)` in `DESC`.

Here is an example:
{{{#!python

item = Item.objects.create()
ItemValue.objects.bulk_create([
    ItemValue(item=self.item, sort_order=0),
    ItemValue(item=self.item),
    ItemValue(item=self.item),
])

sort_order = item.items.all()[0].sort_order  # is 0

item.items.last()

sort_order = item.items.all()[0].sort_order # is None

item.items.last()

sort_order = item.items.all()[0].sort_order # is 0
}}}

The biggest issue with this, is if this is ran into a view or a test, the ordering will always be reversed until we restart the django worker, or if we run `item.items.last()` again to flip the switch.

Here is a full example:
{{{#!python
# models
from django.db import models
from django.db.models import F


class Item(models.Model):
    pass


class ItemValue(models.Model):
    sort_order = models.IntegerField(unique=True, null=True)
    item = models.ForeignKey(Item, on_delete=models.CASCADE, related_name=""items"")

    class Meta:
        ordering = (F(""sort_order"").asc(nulls_last=True), ""id"")


# test
class TestItemValue(TestCase):
    def setUp(self) -> None:
        self.item = Item.objects.create()
        ItemValue.objects.bulk_create([
            ItemValue(item=self.item, sort_order=0),
            ItemValue(item=self.item),
            ItemValue(item=self.item),
        ])

    def test_valid(self):
        self.assertEqual(self.item.items.all()[0].sort_order, 0)

    def test_switch(self):
        self.item.items.last()

        with CaptureQueriesContext(connection) as ctx:
            sort_order = self.item.items.all()[0].sort_order
            sql = ctx[0]['sql']
            order_by = sql[sql.find(""ORDER BY""):]
            self.assertEqual(sort_order, 0, order_by)
}}}

This is an example of the switch in action during a `.all()`:
[[Image(https://i.imgur.com/lVH1pOs.gif)]]"	Bug	closed	Database layer (models, ORM)		Normal	duplicate			Unreviewed	0	0	0	0	0	0