order_by() with expressions crashes on union() querysets.

[Laurent Tramoy]

I have read the recent tickets about unions and order_by (#31496, #27995, #30628) , and my bug is slightly different, so I hope it's not a duplicate.

Let's consider two similar models:

class EntityA(models.Model):
    name_a = models.CharField(max_length=128, null=True)
    dt_a = models.DateTimeField(null=True)


class EntityB(models.Model):
    name_b = models.CharField(max_length=128, null=True)
    dt_b = models.DateTimeField(null=True)

EntityA.objects.create(name_a="a")
EntityA.objects.create(name_a="qwerty", dt_a=timezone.now())
EntityB.objects.create(name_b="random", dt_b=timezone.now())
EntityB.objects.create(name_b="b")

qs_a = EntityA.objects.values(name=F("name_a"), dt=F("dt_a"))
qs_b = EntityB.objects.values(name=F("name_b"), dt=F("dt_b"))

# union queryset
queryset = qs_a.union(qs_b)

I can use a simple ORDER BY clause:

queryset.order_by("-dt")

And everything will work, no problem here.

What I actually want is the same query, but with a NULLS LAST
Usually the query becomes:

queryset.order_by(F("dt").desc(nulls_last=True)) 

but that raises a

DatabaseError: ORDER BY term does not match any column in the result set.

I know unions can handle only a few clauses, but ORDER BY is one of them, so I'm unsure whether this is the expected behaviour or not.
If it's expected, then the raised exception could be more explicit.

[Mariusz Felisiak]

Thanks, yes it is a different issue that is strictly related with using expressions in .order_by(). Potential fix should target ​these lines. Would you like to try?

[Laurent Tramoy]

Replying to felixxm:

Thanks, yes it is a different issue that is strictly related with using expressions in .order_by(). Potential fix should target ​these lines. Would you like to try?

Ok I'll give it a try this week end, I just need to read django conventions before, as I've never committed on this project

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 51ad767:

Refs #31614 -- Added test for aliases ordering on combined querysets.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2aac176e:

Fixed #31614 -- Fixed aliases ordering by OrderBy() expressions of combined queryset.

[Mariusz Felisiak <felisiak.mariusz@…>]

In df88f24:

[3.1.x] Fixed #31614 -- Fixed aliases ordering by OrderBy() expressions of combined queryset.

Backport of 2aac176e86204785f0f2ec4838049d8fed70870e from master

[GitHub <noreply@…>]

In f4bab098:

Refs #31614 -- Added test for ordering by OrderBy() of combined queryset with not selected columns.