Opened 7 years ago

Last modified 7 years ago

#29016 closed Bug

Reuse of UpdateQuery breaks delete some updates — at Initial Version

Reported by: Étienne Loks Owned by: nobody
Component: Database layer (models, ORM) Version: 1.11
Severity: Release blocker Keywords:
Cc: Triage Stage: Ready for checkin
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

On a model A, when deleting a foreign key pointing to a model B, some other foreign key of the model A pointing to the same model B may be nullified.

I have isolated this behaviour on a simple project:

models.py:

from django.db import models


class ChildModel(models.Model):
    name = models.CharField(max_length=200)


class ParentModel(models.Model):
    name = models.CharField(max_length=200)
    child_1 = models.ForeignKey(ChildModel, on_delete=models.SET_NULL,
                                related_name='parents_1', null=True)
    child_2 = models.ForeignKey(ChildModel, on_delete=models.SET_NULL,
                                related_name='parents_2', null=True)

Django shell session:

from testapp.models import ParentModel, ChildModel

child_1 = ChildModel.objects.create(name="child_1")
child_2 = ChildModel.objects.create(name="child_2")
parent_1 = ParentModel.objects.create(name="parent 1", child_1=child_1, child_2=child_2)
parent_2 = ParentModel.objects.create(name="parent 2", child_1=child_2, child_2=child_1)

child_1.delete()
parent_1 = ParentModel.objects.get(pk=parent_1.pk)
parent_2 = ParentModel.objects.get(pk=parent_2.pk)
# parent_1.child_2 and parent_2.child_1 should be normaly equal to child_2 but...
parent_1.child_2 is not None and parent_2.child_1 is not None
# False is returned

This simple project has been tested on an SQLite database. The same behaviour has been first discovered on a PostgreSQL database.

A mis-reuse of an UpdateQuery seems to be the cause of this bug.
The attached patch fixes the issue.

After search on the django bug tracker I have found another issue with the same patch attached #28099.
I have opened this new ticket because the issue seems to be more severe (I have experienced large data loss) and more general.

This issue has been found on version 1.11 and 2.0 of Django.

Change History (1)

by Étienne Loks, 7 years ago

Attachment: fix_delete_on_update.patch added

Simple patch version (no regression test yet)

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