#33682: SQL generation bug in `.distinct()` when supplied fields go through
multiple many-related tables
-------------------------------------+-------------------------------------
Reporter: Robert Leach | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 3.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: sql, distinct, | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Mariusz Felisiak):
Replying to [comment:5 Robert Leach]:
> When those methods are assessed individually, I understand why those
fields are the preferred solution (e.g. the meta ordering may not be
unique), but given that `distinct` requires the same fields be present at
the beginning of the order-by, I don't know what prevents the code to be
written to have those fields be resolved in a way that is copacetic.
Like, why not convert the reference into 2 additional fields that
together, meet both requirements (`name` AND `compound_id`)? Order-by
would be satisfied and distinct would be satisfied. Or... in my case,
`name` is unique, so distinct could resolve to the meta ordering without
issue...
>
> Is there a technical reason the code doesn't already do this?
This would be another logic that's implicit and probably unexpected by
users (at least in some cases). As far as I'm aware it's preferable to
fail loudly even with a `ProgrammingError`. I'm not sure how to improve
this note, maybe it's enough to add a correct example:
{{{#!diff
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index a9da1dcf7e..891b8255b0 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -565,7 +565,9 @@ Examples (those after the first will only work on
PostgreSQL)::
...wouldn't work because the query would be ordered by ``blog__name``
thus
mismatching the ``DISTINCT ON`` expression. You'd have to explicitly
order
by the relation ``_id`` field (``blog_id`` in this case) or the
referenced
- one (``blog__pk``) to make sure both expressions match.
+ one (``blog__pk``) to make sure both expressions match::
+
+ Entry.objects.order_by('blog_id').distinct('blog_id')
``values()``
~~~~~~~~~~~~
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/33682#comment:6>
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