#35810: Provide `Select` class for `select_related` (like `Prefetch` exists for
`prefetch_related`)
-------------------------------------+-------------------------------------
Reporter: Bart van Andel | Owner: (none)
Type: New feature | Status: closed
Component: Database layer | Version: 5.1
(models, ORM) |
Severity: Normal | Resolution: wontfix
Keywords: query optimization | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Simon Charette):
* resolution: => wontfix
* status: new => closed
Comment:
I'm struggling to find it now but I know the idea of allow custom classes
to be provided to `select_related` was already discussed somewhere in the
past years.
I think the question here is less about whether we'd want to support such
pattern but how exactly it should be achieved.
For example, for reverse relationships it could be done using `LEFT JOIN
LATERAL` on backends that support it
{{{#!python
chats = Chat.objects.select_related(
Select(
"messages",
queryset=Message.objects.order_by("created_at"),
to_attr="lastest_message",
),
)
}}}
{{{#!sql
SELECT
chat.*,
latest_message.*
LEFT JOIN LATERAL (
SELECT *
FROM message
WHERE chat_id = chat.id
ORDER BY created_at DESC
LIMIT 1
) latest_message
}}}
I'm not sure what the best way to express this through the ORM should be
though. It feels like it could also be something that is expressed through
a `Subquery` that doesn't make use of `values` to limit to a single field
{{{#!python
chats = Chat.objects.annotate(
lastest_message=Message.objects.filter(
chat_id=OuterRef("id"),
).order_by("created_at")[1]
)
}}}
In all cases, just like any new large feature request,
[https://docs.djangoproject.com/en/stable/internals/contributing/bugs-and-
features/#requesting-features it should be discussed on the forum] before
hand to gather consensus.
--
Ticket URL: <https://code.djangoproject.com/ticket/35810#comment:2>
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