#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
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Comment (by Simon Charette):
I did a bit of investigation to determine the length of changes this would
require. I started with trying to make `count()` return a lazy object as
it's a bit easier to implement than `aggregate()`. I didn't implement the
`resolve_expression` or `as_sql` for now.
As expected making `count()` lazy breaks a few tests that expect the query
to be immediately executed but but only a few adjustments to `LazyObject`
are required to get most of the suite passing.
Still this breaks backward compatiblity and would certainly break more
than just test code out there. For example, a view a could be getting a
count, perfoming some alterations and retrieving a new count and that
would break if the first query is not immediately executed.
The work I've done can be found here
https://github.com/django/django/compare/master...charettes:ticket-28296
What I suggest we do here instead is to introduce a new
`AggregateSubQuery(queryset, aggregate)` expression instead. It's
certainly not as elegant as using `count()` or `aggregate()` but it does
maintain backward compatiblity.
e.g.
{{{#!python
Performance.objects.annotate(
reserved_seats=AggregateSubQuery(SeatReservation.objects.filter(
performance=OuterRef(name='pk'),
status__in=TAKEN_TYPES,
), Count('pk')),
)
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/28296#comment:10>
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