Opened 6 years ago

Closed 5 years ago

Last modified 5 years ago

#30325 closed Bug (fixed)

Inconsistent count() behaviour on reverse relations in ManyToManyFields with custom model manager

Reported by: Tobias Kunze Owned by: Tobias Kunze
Component: Database layer (models, ORM) Version: 2.2
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

When looking up a reverse relation on a ManyToManyField that uses a custom model manager, not using all() before calling count() will not use the custom model manager. This results in obj.m2m_lookup.count() yielding a different result than obj.m2m_lookup.all().count, which is new behaviour in 2.2, and unexpected (at least to me).

I've written up a minimal test case to showcase the issue. Inlining here:

from django.db import models


class Author(models.Model):
    name = models.CharField(max_length=100)


class BookManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().exclude(state='deleted')


class Book(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(to=Author, related_name='books')
    state = models.CharField(choices=(('regular', 'regular'), ('deleted', 'deleted')), default='regular', max_length=100)

    objects = BookManager()

a = Author.objects.create(name='Bill')
b1 = Book.objects.create(title='Sonnets', state='deleted')
b2 = Book.objects.create(title='Hamlet')
b1.authors.add(a)
b2.authors.add(a)
a.books.count()  # Returns 2
a.books.all().count()  # Returns 1

Change History (12)

comment:1 by Pizzolato Davide, 6 years ago

Resolution: invalid
Status: newclosed

a.books.all() is managed by the BookManager so it exclude the 'deleted' book b1

comment:2 by Mariusz Felisiak, 6 years ago

Resolution: invalid
Severity: NormalRelease blocker
Status: closednew
Triage Stage: UnreviewedAccepted

Reproduced at 1ffddfc233e2d5139cc6ec31a4ec6ef70b10f87f and works in Django 2.1.

Thanks for the report. Can you bisect to check which commit change this behavior?

comment:3 by Pizzolato Davide, 6 years ago

Resolution: invalid
Status: newclosed

This is not a bug!

comment:4 by Mariusz Felisiak, 6 years ago

Resolution: invalid
Status: closednew

PizzolatoDavide Please do not shout.

a.books.count() != a.books.all().count() for me it is a bug, moreover this behavior has changed in Django 2.2.

comment:5 by Tobias Kunze, 6 years ago

This is not a bug!

Could you elaborate on this? As it is changed behaviour from Django 2.1 and previous versions, it's either a bug/regression, or a documentation bug by not mentioning this changed behaviour in the release notes, in my opinion. Up until now I've assumed that something.related.count() and something.related.all().count() will yield the same result, and if this is not true, it should be documented. (Or is it documented? Could you point me at the right place to look?)

comment:6 by Marten Kenbeek, 6 years ago

Regression caused by 1299421 (#29725). .count() and .exists() on a many-to-many relation now use the through model's base manager instead of the related model's default manager.

comment:7 by Tobias Kunze, 5 years ago

As per discussion at DjangoCon Europe, the patch in question will probably be reverted, closing this issue (and re-opening #29725, potentially). I've submitted regression tests: https://github.com/django/django/pull/11205

comment:8 by Mariusz Felisiak, 5 years ago

Has patch: set
Owner: changed from nobody to Tobias Kunze
Status: newassigned
Triage Stage: AcceptedReady for checkin

comment:9 by Mariusz Felisiak <felisiak.mariusz@…>, 5 years ago

Resolution: fixed
Status: assignedclosed

In 5f7991c4:

Fixed #30325 -- Reverted "Fixed #29725 -- Removed unnecessary join in QuerySet.count() and exists() on a many-to-many relation."

This reverts commit 1299421cadc4fcf63585f2f88337078e43e660e0 due to
a regression with custom managers.

comment:10 by Mariusz Felisiak <felisiak.mariusz@…>, 5 years ago

In 9ac8520:

Refs #30325 -- Added tests for using count()/exists() with custom managers and reverse M2M relations.

comment:11 by Mariusz Felisiak <felisiak.mariusz@…>, 5 years ago

In e8de1cc9:

[2.2.x] Fixed #30325 -- Reverted "Fixed #29725 -- Removed unnecessary join in QuerySet.count() and exists() on a many-to-many relation."

This reverts commit 1299421cadc4fcf63585f2f88337078e43e660e0 due to
a regression with custom managers.

Backport of 5f7991c42cff73b6278106d499d719b726f85ead from master

comment:12 by Mariusz Felisiak <felisiak.mariusz@…>, 5 years ago

In 5289d4f:

[2.2.x] Refs #30325 -- Added tests for using count()/exists() with custom managers and reverse M2M relations.

Backport of 9ac8520fcde29840a1345be19d80dbda53aa6d03 from master

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