#18676: Django should do m2m deletes in a single query when possible
-------------------------------------+-------------------------------------
Reporter: jdunck | Owner: nobody
Type: | Status: closed
Cleanup/optimization | Version: 1.4
Component: Database layer | Resolution: fixed
(models, ORM) | Triage Stage: Ready for
Severity: Normal | checkin
Keywords: | Needs documentation: 0
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
-------------------------------------+-------------------------------------
Comment (by pressureman):
After doing a git bisect, I discovered that the commit
1cd6e04cd4f768bcd4385b75de433d497d938f82 caused a regression in an
existing project of mine, that uses a custom object manager. The setup is
as follows (simplified - other models also have `ForeignKey` back to
`OrgUnit` model):
{{{
#!python
class OrgUnit(models.Model):
name = models.CharField(max_length=64, unique=True)
groups = models.ManyToManyField(Group, blank=True)
users = models.ManyToManyField(User, blank=True)
class LoginManager(models.Manager):
def restricted(self, user):
q_group = Q(orgunit__groups__user=user)
q_user = Q(orgunit__users=user)
return self.filter(q_group | q_user).distinct()
class Login(models.Model):
description = models.CharField(max_length=32)
orgunit = models.ForeignKey(OrgUnit)
private_data = models.TextField(editable=False)
objects = LoginManager()
}}}
My view POSTs a list of ids to a view that deletes Login objects, and it
throws a DB exception "subquery has too many columns". I can see quite
clearly from the generated SQL that the sub-select is including
unnecessary fields for the "DELETE FROM foo WHERE id in (SELECT id,
description FROM login ... )". I don't understand why the sub-select is
including the `Login.description` field :-?
The following is the code in the view that triggers the exception:
{{{
#!python
id_list = map(int, request.POST.getlist('_id_list'))
Login.objects.restricted(request.user).filter(id__in=id_list).delete()
}}}
What I've found is that if I modify the `LoginManager.restricted()` to
either not `filter()` (undesirable) or not `.distinct()` then the bug does
not occur.
This code is a few years old now, and I don't remember why exactly I put a
`.distinct()` in there. It doesn't seem to cause any undesirable side
effects if I remove it, but I'd like to try to understand why this Django
commit has caused this regression in the first place, as it may affect
other people.
--
Ticket URL: <https://code.djangoproject.com/ticket/18676#comment:8>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To post to this group, send email to django-updates@googlegroups.com.
To unsubscribe from this group, send email to
django-updates+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.
