Model.delete() issues extra queries after a deferred queryset

[Pantelis Petridis]

Consider the following models

from django.db import models

class ModelA(models.Model):
        a = models.CharField(max_length=15)

class ModelB(models.Model):
        fk = models.ForeignKey(ModelA)

and the following test

from django.test import TestCase
from .models import ModelA

class DeferProblemTestCase(TestCase):
        def test_defer(self):
                a = ModelA.objects.create(a='test')
                with self.assertNumQueries(2):
                        a.delete()

                unrelated_qs = list(ModelA.objects.defer('a'))

                # let's try once again
                a = ModelA.objects.create(a='test')
                with self.assertNumQueries(2):
                        a.delete()

The above test fails under django 1.8.6, as the second call to .delete() will produce 3 queries instead of 2. It seems django somehow caches the deferred model and tries to delete any related ModelB records twice. Except from the overhead, this can lead to random test fails based on the execution order.

[Simon Charette]

This has been in the process of solving #18012 (8bdfabed6563f2ae136ad43e05bb254c9c15811a).

I guess adding this regression test case to tests/delete wouldn't hurt at this point.

[Pantelis Petridis]

Is there any chance this gets backported to django 1.8, since #18012 is a Cleanup/optimization but this one's a bug?

[Simon Charette]

I don't think back porting should apply here since this isn't a bug in a new feature.

[Pantelis Petridis]

I came up across this issue while upgrading from django 1.7 to 1.8. I can confirm the above test works in 1.7.XX and the issue was introduced in 1.8. But since I'm not familiar with the release cycle I don't know if that would justify a back port. Thanks anyway!

[Simon Charette]

If it's a regression in 1.8 then I think a fix should be backported.

I think we should avoid backporting the #18012 related patches and provide a more localized fix.

[Tim Graham]

As far as I can tell, there isn't a patch to review yet.

[Simon Charette]

​PR

[Simon Charette <charette.s@…>]

In 7c3ef19:

[1.8.x] Fixed #25685 -- Fixed a duplicate query regression on deletion of proxied models.

Thanks to Trac alias ppetrid for the report and Tim for the review.

[Simon Charette <charette.s@…>]

In eb7a329:

[1.9.x] Fixed #25685 -- Fixed a duplicate query regression on deletion of proxied models.

Thanks to Trac alias ppetrid for the report and Tim for the review.

Conflicts:

tests/delete/tests.py

Forward port of 7c3ef19978b36b61db88a519f799f1ce8d019679 from stable/1.8.x

[Simon Charette <charette.s@…>]

In 6d03bc1:

Fixed #25685 -- Fixed a duplicate query regression on deletion of proxied models.

Thanks to Trac alias ppetrid for the report and Tim for the review.

Conflicts:

django/db/models/deletion.py
tests/delete/tests.py

Forward port of 7c3ef19978b36b61db88a519f799f1ce8d019679 from stable/1.8.x