#6422: Support for 'DISTINCT ON' queries with QuerySet.distinct()
-------------------------------------+-------------------------------------
Reporter: Manfred | Owner: jgelens
Wassmann <manolo@…> | Status: assigned
Type: New | Component: Database layer
feature | (models, ORM)
Milestone: | Severity: Normal
Version: SVN | Keywords: dceu2011
Resolution: | Has patch: 1
Triage Stage: Accepted | Needs tests: 0
Needs documentation: 0 | Easy pickings: 0
Patch needs improvement: 0 |
UI/UX: 0 |
-------------------------------------+-------------------------------------
Changes (by jgelens):
* needs_better_patch: 1 => 0
Comment:
The 'parent' is an foreign key to self. When you print the query you'll
see it doesn't add the parent_id to the ORDER BY clause:
{{{
>>> print Tag.objects.all().order_by('parent').query
SELECT "queries_tag"."id", "queries_tag"."name",
"queries_tag"."parent_id", "queries_tag"."category_id" FROM "queries_tag"
LEFT OUTER JOIN "queries_tag" T2 ON ("queries_tag"."parent_id" = T2."id")
ORDER BY T2."name" ASC
}}}
The DISTINCT ON expression requires the leftmost ORDER BY to match the
fields given in the DISTINCT ON clause. To make sure the parent_id will be
in ORDER BY, use parent__pk or parent__id:
{{{
>>> print Tag.objects.all().order_by('parent__pk').query
SELECT "queries_tag"."id", "queries_tag"."name",
"queries_tag"."parent_id", "queries_tag"."category_id" FROM "queries_tag"
LEFT OUTER JOIN "queries_tag" T2 ON ("queries_tag"."parent_id" = T2."id")
ORDER BY "queries_tag"."parent_id" ASC
}}}
Now if you update your ORM call a lil, you'll get the expected results:
{{{
>>> print Tag.objects.distinct('parent').order_by('parent__pk')
[<Tag: t3>, <Tag: t5>, <Tag: t1>]
}}}
I added a note to the documentation about the 'order_by' requirement.
It's not a bug, so I hope the documentation is more clear now.
--
Ticket URL: <https://code.djangoproject.com/ticket/6422#comment:37>
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