[Django] #37258: Combining `values()` querysets on a model with `Meta.ordering` crashes

[Django]

#37258: Combining `values()` querysets on a model with `Meta.ordering` crashes
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Reporter: Adam | Owner: Adam Johnson
Johnson |
Type: Bug | Status: assigned
Component: Database | Version: 6.1
layer (models, ORM) |
Severity: Release | Keywords:
blocker |
Triage Stage: | Has patch: 1
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Combining querysets restricted with {{{values()}}}/{{{values_list()}}}
(e.g. via {{{union()}}}) on a model with {{{Meta.ordering}}} crashes at
SQL compilation on Django 6.1 when the default ordering columns are not
part of the selected columns:

{{{
django.db.utils.DatabaseError: ORDER BY term does not match any column in
the result set.
}}}

Minimal reproduction (crashes on PostgreSQL, MySQL, and Oracle — any
backend with {{{supports_slicing_ordering_in_compound=True}}}; the same
code works on Django 6.0):

{{{#!python
class Book(models.Model):
name = models.CharField(max_length=10)
rank = models.IntegerField()

class Meta:
ordering = ["rank"]

Book.objects.values("name").union(Book.objects.values("name")) # crashes
on iteration
}}}

Same for {{{values_list()}}}, and for
{{{intersection()}}}/{{{difference()}}}.
{{{qs1.union(qs2).values("name")}}} doesn't crash but silently adds a
spurious {{{__orderbycol2}}} column to the compiled query and result set.

Regression in 087bb9e8f3478d53f12b1737af865992af17c5f2 (Refs #36644 --
Applied default ordering after union()), which sets {{{default_ordering =
True}}} on the combined query so that {{{Meta.ordering}}} is applied after
the combination. Root cause: for combined queries with a limited list of
selected fields, {{{SQLCompiler.get_order_by()}}} cannot inject the
implicit ordering columns into the {{{values()}}} parts
({{{has_select_fields}}} guard in {{{django/db/models/sql/compiler.py}}}),
so it raises {{{DatabaseError}}}.
--
Ticket URL: <https://code.djangoproject.com/ticket/37258>
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