#23518 closed Cleanup/optimization (fixed)
Subquery does not respect explicit __exact
Reported by: | john-parton | Owned by: | nobody |
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Component: | Database layer (models, ORM) | Version: | 1.7 |
Severity: | Normal | Keywords: | subquery subselect exact |
Cc: | Triage Stage: | Accepted | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
I think it should be possible to use a subquery as an expression with __exact
.
It appears that when you perform a subquery, the ORM includes an implicit __in
. This is fine, because I'm sure that covers most use cases, but if you try to provide an explicit __exact
, the ORM disregards it.
# models.py from django.db import models class Child(models.Model): parent = models.ForeignKey('Parent') class Parent(models.Model): slug = models.SlugField(unique=True)
In [1]: from model_test.models import Child, Parent In [2]: parent = Parent(slug='test') In [3]: parent.save() In [4]: child = Child(parent=parent) In [5]: child.save() In [6]: children = Child.objects.filter(parent_id=Parent.objects.filter(slug='test')) In [7]: print(children.query) SELECT "model_test_child"."id", "model_test_child"."parent_id" FROM "model_test_child" WHERE "model_test_child"."parent_id" IN (SELECT U0."id" FROM "model_test_parent" U0 WHERE U0."slug" = test) In [8]: children = Child.objects.filter(parent_id__exact=Parent.objects.filter(slug='test')) In [9]: print(children.query) SELECT "model_test_child"."id", "model_test_child"."parent_id" FROM "model_test_child" WHERE "model_test_child"."parent_id" IN (SELECT U0."id" FROM "model_test_parent" U0 WHERE U0."slug" = test)
The expected output of the 9th command is:
SELECT "model_test_child"."id", "model_test_child"."parent_id" FROM "model_test_child" WHERE "model_test_child"."parent_id" = (SELECT U0."id" FROM "model_test_parent" U0 WHERE U0."slug" = 'test');
Obviously using __exact
isn't always possible, but because Parent
has a unique slug, we can guarantee that the subquery returns one or zero rows. Alternative, we could have added a LIMIT 1
(or a [:1]
with the ORM) to ensure that it has exactly one or zero rows.
The most simple way to do error handling would to just wrap the ProgrammingError: more than one row returned by a subquery used as an expression
which would get raised by the connection.
At the very least, shouldn't the __exact
transform raise some error or issue some warning that it does nothing?
Thoughts?
Change History (7)
comment:1 by , 10 years ago
comment:2 by , 10 years ago
It looks like the ORM automatically considers that you are doing an __in
lookup when you give a QuerySet
as the right-hand side of a filter on a related field, whatever the lookup type is (see https://github.com/django/django/blob/bbc3505ef81768aa2afac8f73e6d45b5e8000c55/django/db/models/fields/related.py#L1607).
For instance:
Author.objects.filter(item__startswith=Item.objects.all()) [<Author: a1>, <Author: a2>, <Author: a2>, <Author: a4>]
Maybe it would be more consistent if the ORM raised some error instead ?
That being said, I agree that .get()
is better when you expect a single result.
comment:3 by , 10 years ago
Triage Stage: | Unreviewed → Accepted |
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Type: | Uncategorized → Cleanup/optimization |
comment:4 by , 8 years ago
Resolution: | → fixed |
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Status: | new → closed |
follow-up: 6 comment:5 by , 8 years ago
Thanks for the feedback.
For anyone who might come back to this ticket at a later date:
The reason someone might not want to use .get()
is because it executes another SQL query.
Parent.objects.filter(parent__slug='test')
is not the same query. It uses a join instead of a subquery, and might have different performance characteristics. Often I refactor a SQL query looking at the output of EXPLAIN
, and then try to recreate the query using the ORM.
comment:6 by , 8 years ago
Replying to john-parton:
Parent.objects.filter(parent__slug='test')
is not the same query. It uses a join instead of a subquery, and might have different performance characteristics. Often I refactor a SQL query looking at the output ofEXPLAIN
, and then try to recreate the query using the ORM.
Whether a specific ORM expression uses a join or a subquery is an implementation detail which you should not count on; these things are allowed to change (and have changed in the past) without notice, so long as the semantics of the query is preserved. The performance characteristics of a given SQL query, as well, may change between different databases and even between versions of the same database product. If your requirements really dictate that you use one form rather than the other, you should be using raw SQL.
comment:7 by , 8 years ago
I think with the release of version 1.11, it is now possible to do this without using raw SQL.
https://docs.djangoproject.com/en/1.11/releases/1.11/#subquery-expressions
If you expect the query to return one result, why not use
.get()
instead of.filter()
. Also couldn't you restructure your example more simply asParent.objects.filter(parent__slug='test')
. I guess some documentation could at least be updated.