#35751: Ordering a model via a m2m field creates unintended side effect for
ForeignKeys
-------------------------------------+-------------------------------------
Reporter: Dennis Scheiba | Owner: (none)
Type: Bug | Status: closed
Component: Database layer | Version: 5.1
(models, ORM) |
Severity: Normal | Resolution: invalid
Keywords: ORM ordering | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Simon Charette):
I personally think that `order_by(multivalued__value)` is a foot-gun due
to its ambiguity and duplicate row spanning behaviour and should be
deprecated but it poses some challenges as it is a behavior that was
supported and even documented for over a decade so it's safe to assume
that some users might be legitimately depend on it for some nice query
needs (e.g. adding `distinct()` afterwards to deal with the duplicate row
issue).
If we were to raise warnings or consider deprecating this feature we
should have an upgrade path that covers these use cases. In your
particular case
{{{#!python
OrderItem.objects.order_by("booking_times__date").distinct()
}}}
which results in
{{{#!sql
SELECT DISTINCT orderitem.*
FROM orderitem
LEFT JOIN orderitem_booking_times ON (orderitem_booking_times.orderitem_id
= orderitem.id)
LEFT JOIN bookingtime ON (orderitem_booking_times.bookingtime_id =
bookingtime.id)
ORDER BY bookingtime.date
}}}
can be replaced with
{{{#!python
OrderItem.objects.order_by(Min("booking_times__date"))
}}}
which results in
{{{#!sql
SELECT orderitem.*
FROM orderitem
LEFT JOIN orderitem_booking_times ON (orderitem_booking_times.orderitem_id
= orderitem.id)
LEFT JOIN bookingtime ON (orderitem_booking_times.bookingtime_id =
bookingtime.id)
GROUP BY orderitem.id
ORDER BY MIN(bookingtime.date)
}}}
Since `order_by` is resolved late in the query compilation process it
should be doable to present a deprecation warning that suggests the usage
of `Min` and `Max` instead.
--
Ticket URL: <https://code.djangoproject.com/ticket/35751#comment:6>
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