#25685: Deferred model leak
-------------------------------------+-------------------------------------
Reporter: ppetrid | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.8
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by ppetrid):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Old description:
> Consider the following models
>
> {{{
> from django.db import models
>
> class ModelA(models.Model):
> a = models.CharField(max_length=1)
>
> 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'))
>
> # lets'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.
New description:
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.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/25685#comment:1>
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