#33608: select_related() to nullable FK with db_constraint=False do not promote
OUTER joins.
-------------------------------------+-------------------------------------
Reporter: Mathieu Hinderyckx | Owner: nobody
Type: Bug | Status: closed
Component: Database layer | Version: 3.2
(models, ORM) |
Severity: Normal | Resolution: invalid
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 Mathieu Hinderyckx):
Replying to [comment:3 Simon Charette]:
> The question here is should the ORM account for possible integrity
violations in the first place, lets not forget that `db_constraint` was
basically introduced as
[https://github.com/django/django/commit/b55cde054ee7dd22f93c3522a8ddb1d04193bcac
some form of shadow option] without in-dept discussion about its desired
behaviour with regards to violations.
>
> How the ORM should account for possible integrity violation was already
discussed in the past plenty when usage of the MySQL's MyISAM backend (and
SQLite in older versions) which doesn't support foreign keys was more
common and caused users to run into this exact issue.
>
> Given we've opted not to special case JOIN'ing logic for
`supports_foreign_keys` in the past I don't see why we should take a
different stance for `db_constraint=False`.
>
> If we allowed
`Object.objects.filter(remote_object_id=1234).select_related('remote_object')`
to return `Object` instances what would their `.remote_object` be? A mock
`RemoteObject(id=1234)` object? Simply `None`? Aren't both approaches
breaking symmetry with
`Object.objects.filter(remote_object_id=1234).first().remote_object`
raising a `RemoteObject.DoesNotExist` exception?
`Object.objects.filter(remote_object_id=1234).first().remote_object`
doesn't raise a DoesNotExist exception. It returns null, as it should be.
First it fetches the Object with `remote_object_id=1234` which exists.
Secondly, It then accesses the FK field on that Object pointing to a
RemoteObject, which is null as it does not exist in the remote table
(hence why the db_constraint=False). That works as expected, just like an
'ordinary' FK. What doesn't work as expected is that adding a
select_related() breaks the filter statement. Select_related is an
optimization, the expectation is that it does the same two steps as before
but with a single DB query. That isn't the case currently; it silently
breaks the filtering logic, and doesn't event return the Object at all.
--
Ticket URL: <https://code.djangoproject.com/ticket/33608#comment:5>
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