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
37466	"order_by(""pk"") on a union() of a composite pk model crashes or sorts wrongly"	Akshat Gupta		"{{{#!python
class Comment(models.Model):
    pk = models.CompositePrimaryKey(""tenant_id"", ""comment_id"")
    tenant_id = models.IntegerField()
    comment_id = models.IntegerField()

# rows (tenant_id, comment_id): (1, 2), (2, 1), (1, 1)
qs = Comment.objects.filter(tenant_id=1).union(Comment.objects.filter(tenant_id=2))
}}}

Ordering a combined query by a composite primary key fails in two ways.

1. On a normal queryset, `order_by(""pk"")` crashes. `first()` crashes too, because it orders by `pk`.

{{{#!python
qs.first()
qs.order_by(""pk"")  # when evaluated
# TypeError: 'NoneType' object does not support item assignment
}}}

The error is raised in `Query.add_select_col()` (called from `SQLCompiler.get_order_by()`), because `query.selected` is `None`.

2. With `values_list(""pk"")` there is no error, but the order is wrong.

{{{#!python
list(qs.values_list(""pk"").order_by(""pk""))
# [((1, 1),), ((2, 1),), ((1, 2),)]
# expected: [((1, 1),), ((1, 2),), ((2, 1),)]
}}}

The SQL ends with `ORDER BY ""col1""`, and `col1` is only the last pk column (`comment_id`). The same query without `union()` is ordered correctly.

I think `get_order_by()` treats the composite pk (`ColPairs`) as a single column in the combinator branch. It should order by each column of the pk.

Related: #36065, #37354 (other composite pk ordering bugs).
"	Bug	new	Database layer (models, ORM)	dev	Normal		CompositePrimaryKey, union, order_by	Akshat Gupta	Unreviewed	0	0	0	0	0	0