#28296: Add support for aggregation through subqueries
-------------------------------------+-------------------------------------
Reporter: László Károlyi | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Simon Charette):
* version: 1.11 => master
* type: Bug => New feature
* stage: Unreviewed => Accepted
Comment:
Hello László, thanks for taking the time to provide all these details.
I think that what you'd like to do use is the `aggregate` method instead
of the `annotate` one to avoid the group by. Something along the lines of
{{{#!python
Performance.objects.annotate(
reserved_seats=Subquery(
SeatReservation.objects.filter(
performance=OuterRef(name='pk'),
status__in=TAKEN_TYPES,
).aggregate(Count('pk'))
),
)
}}}
Unfortunately the `Subquery` API
[https://github.com/django/django/pull/6478#issuecomment-217759006 doesn't
allow that yet].
It would require to either make `aggregate` return a lazy object that
`Subquery` can deal with (right now it returns a `dict` on call) which has
the potential of breaking backward compatibility or introducing a new kind
of expression to deal with this case (e.g. `AggregateSubquery(query,
Count('pk'))`).
In an ideal world we'd be able to call `count()` directly and avoid the
`Subquery` wrapper
{{{#!python
Performance.objects.annotate(
reserved_seats=SeatReservation.objects.filter(
performance=OuterRef(name='pk'),
status__in=TAKEN_TYPES,
).count()
)
}}}
I'm converting the ticket to a feature request for subquery aggregation.
It has a bit of overlap with #10060 as it would add a way to work around
the issue by explicitly using subqueries as opposed to having the ORM do
automatic pushdown but I still consider this a feature on it's own.
--
Ticket URL: <https://code.djangoproject.com/ticket/28296#comment:3>
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