#24218: Use sub-query in ORM when distinct and order_by columns do not match
-------------------------------------+-------------------------------------
Reporter: miki725 | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: subquery distinct | Triage Stage: Accepted
order_by |
Has patch: 1 | Needs documentation: 1
Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by carljm):
@miki725 - In a case like yours, I've generally just done `.distinct()`
instead of `.distinct('id')`. Since you're only selecting columns from
foo, that should give the same results, and it makes Postgres happy with
any `ORDER BY` clause you like, with no need for a subquery. So I'm not
sure that your particular case actually offers a use case for this feature
- in this case our support for `DISTINCT ON` is actually an "attractive
nuisance."
@akaariai - that API doesn't feel quite right to me. `order_by()` in every
other case determines the ordering of the returned results. Just because
in this case we are also applying a SQL `ORDER BY` clause in the subquery
(an implementation detail) doesn't mean that overloading `order_by()` is
the right API for it. For addressing this particular use case, I think a
more intuitive API would be to introduce an ordering argument to
`.distinct()` to allow customization of the ordering used to pick the
distinct row. Of course, that's a less general API - but I think we would
need a more thorough list of the cases where subqueries are needed in
order to see what a generic subqueries API should look like (e.g. are you
intending to limit this API to cases where the outer query is simply a
`SELECT *` from the subquery?). It feels to me that a generic subqueries
API should involve passing one queryset to another, or wrapping one in
another, much like the current case where passing a queryset to an `__in`
filter results in a subquery.
--
Ticket URL: <https://code.djangoproject.com/ticket/24218#comment:8>
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