#19580 closed Cleanup/optimization (fixed)
Unify reverse foreign key and m2m unsaved model querying
Reported by: | Anssi Kääriäinen | Owned by: | raydeal |
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Component: | Database layer (models, ORM) | Version: | dev |
Severity: | Normal | Keywords: | |
Cc: | olivier.tabone@…, Matt Westcott | Triage Stage: | Ready for checkin |
Has patch: | yes | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | yes | UI/UX: | no |
Description
Currently when querying unsaved reverse relations the behavior differs.
Using model:
class Foo(models.Model): fk = models.ForeignKey('self', related_name='fk_rev') m2m = models.ManyToManyField('self')
and test case:
print(Foo().fk_rev.all()) print(Foo().m2m.all())
We get [] from the first filter, but an error
ValueError: "<Foo: Foo object>" needs to have a value for field "from_foo" before this many-to-many relationship can be used.
from the second filter.
So, m2m fields can't be filtered if the object isn't saved, but reverse fk fields can be filtered.
There is a (slightly stale) patch for #17541 which makes fk fields and m2m fields work consistently. The changes in behavior are:
* Nullable many-to-many and foreign key relations will return an empty queryset when the relation field is null. For many-to-many this was previously an error (no change for foreign keys). * Trying to add objects to a foreign key relation when the relation field is null is now an error (no change for m2m). * Trying to use a relation of any kind when the object isn't saved is now an error (no change for m2m).
I think we can squeeze these changes in as bug-fixes. These are slight backwards compatibility changes, but to me it seems that almost always the changes will be visible only in code that isn't working as intended. If these are seen as something likely breaking working code, then I don't see any way to make the APIs consistent.
The #17541 patch is available from: https://github.com/akaariai/django/compare/ticket_17541
Change History (57)
comment:1 by , 12 years ago
Patch needs improvement: | set |
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Triage Stage: | Unreviewed → Design decision needed |
Type: | Uncategorized → Cleanup/optimization |
Version: | 1.4 → master |
comment:2 by , 12 years ago
Triage Stage: | Design decision needed → Accepted |
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Yes, it's better to error loudly on operations that can't work because the database didn't generate an id for the object yet.
comment:3 by , 8 years ago
Easy pickings: | set |
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Summary: | Unify reverse foreign key and m2m querying behavior → Unify reverse foreign key and m2m unsaved model querying |
I think this would allow removing the two lines added in a4c20ae85b40c49e28d1b2227208e4f00d7820df.
Marking as "Easy pickings" since rebasing Anssi's original patch might not be too difficult.
comment:4 by , 8 years ago
Owner: | changed from | to
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Status: | new → assigned |
I will create a new branch, and make the changes made in https://github.com/django/django/compare/master...akaariai:ticket_17541
Will that be correct?
comment:5 by , 8 years ago
Yes, you should rebase the branch and update the release notes for 1.11.
comment:6 by , 8 years ago
Owner: | removed |
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Status: | assigned → new |
comment:7 by , 8 years ago
Hi,
I'm interested in contributing to Django and have been advised to start with some of the 'easy pickings' tasks. I've seen it's been unassigned and is still open, is this something I can pick up?
Cheers,
Michael
comment:8 by , 8 years ago
Owner: | set to |
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Status: | new → assigned |
comment:11 by , 8 years ago
Cc: | added |
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comment:13 by , 8 years ago
Patch needs improvement: | unset |
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comment:14 by , 8 years ago
Patch needs improvement: | set |
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comment:16 by , 8 years ago
Hi there, I was also wondering if further work is needed? From Tim's final comment in the pull request, it seems that most of the changes from Ansi's patch still haven't been updated in master. Thanks
comment:17 by , 8 years ago
Owner: | removed |
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Status: | assigned → new |
I think someone else could pick this up considering we haven't heard from Andrey in a while.
comment:18 by , 8 years ago
Newbie here, but I've begun reading through this problem and would like to take a crack at it. Andrey's pull request seems most of the way there, and if the comments remain valid that gives me a starting point. Thoughts?
comment:20 by , 7 years ago
Owner: | set to |
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Status: | new → assigned |
Please review the latest PR, that is rebased with the master here: https://github.com/django/django/pull/8570
The tests look okay to me. Please suggest any more tests if required. Thanks!
comment:21 by , 7 years ago
Patch needs improvement: | unset |
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comment:23 by , 7 years ago
Patch needs improvement: | unset |
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comment:24 by , 7 years ago
Patch needs improvement: | set |
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comment:25 by , 6 years ago
Hi everyone, I'm interested in contributing to Django, so would like to finish this easy pick if needed. From comments on PR I can see there is only few things left, adjustments to tests and docs. Advise me if there is anything else left.
comment:26 by , 5 years ago
Owner: | removed |
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Status: | assigned → new |
comment:27 by , 5 years ago
Owner: | set to |
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Status: | new → assigned |
comment:28 by , 5 years ago
Patch needs improvement: | unset |
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I've created a PR to fix this issue. Please review and let me know if any change required. Thanks
https://github.com/django/django/pull/11807
comment:29 by , 5 years ago
Needs documentation: | set |
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Needs tests: | set |
comment:34 by , 4 years ago
Need some help with the ticket. PR: https://github.com/django/django/pull/13784.
Thanks!
comment:35 by , 4 years ago
Rebased the previous PR https://github.com/django/django/pull/11807.
I would like to get some clarification on this old ticket, took it under easy-pickings tag.
https://github.com/django/django/pull/11807#pullrequestreview-344544165 - this is the last comment from where I started to search next steps to complete the ticket.
It leads to the undone comments:
Should I move all tests from m2m_through_regress to many_to_many or some particular tests?
I see prepared changelogs in comment https://github.com/django/django/pull/11807#issuecomment-612404721, but don't know where to place them in codebase. Could you explain?
Thanks!
comment:36 by , 4 years ago
Needs documentation: | unset |
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Needs tests: | unset |
PR has been updated https://github.com/django/django/pull/13784
follow-up: 38 comment:37 by , 4 years ago
Patch needs improvement: | set |
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Thanks for this patch, however it's not ready for review. Please fix tests and isort
.
follow-up: 40 comment:38 by , 4 years ago
Replying to Mariusz Felisiak:
Thanks for this patch, however it's not ready for review. Please fix tests and
isort
.
Hello, it seems I need help to fix this error. I'm not sure code from previous contributor is correct and I don't fully understand the subject of this bug (m2m relations).
What is better to do: remove label easy pickings
and try another bug or someone could help me with fixing this bug?
P.S. Please, don't think for nagging. I really don't want to drop the ticket silently and want to continue to contribute at the same time. Thanks in advance with your advice how best to proceed.
comment:39 by , 3 years ago
Owner: | set to |
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I would like to look at code and try to improve the patch.
follow-up: 41 comment:40 by , 3 years ago
Replying to Pavel Druzhinin:
Replying to Mariusz Felisiak:
Thanks for this patch, however it's not ready for review. Please fix tests and
isort
.
Hello, it seems I need help to fix this error. I'm not sure code from previous contributor is correct and I don't fully understand the subject of this bug (m2m relations).
What is better to do: remove labeleasy pickings
and try another bug or someone could help me with fixing this bug?
P.S. Please, don't think for nagging. I really don't want to drop the ticket silently and want to continue to contribute at the same time. Thanks in advance with your advice how best to proceed.
Would you like to carry on with the ticket? I can help you. After reading comments and code my understanding is to make FK reverse working the same way as m2m when id=None, what is done in code, but message is a bit different, it would be good to have similar message in both cases. It will require amend m2m tests as well after update with current django version.
follow-up: 42 comment:41 by , 3 years ago
Replying to raydeal:
Replying to Pavel Druzhinin:
Replying to Mariusz Felisiak:
Thanks for this patch, however it's not ready for review. Please fix tests and
isort
.
Hello, it seems I need help to fix this error. I'm not sure code from previous contributor is correct and I don't fully understand the subject of this bug (m2m relations).
What is better to do: remove labeleasy pickings
and try another bug or someone could help me with fixing this bug?
P.S. Please, don't think for nagging. I really don't want to drop the ticket silently and want to continue to contribute at the same time. Thanks in advance with your advice how best to proceed.
Would you like to carry on with the ticket? I can help you. After reading comments and code my understanding is to make FK reverse working the same way as m2m when id=None, what is done in code, but message is a bit different, it would be good to have similar message in both cases. It will require amend m2m tests as well after update with current django version.
you should take over the ticket and create a new PR as it seems he is not able to fix the issue
comment:42 by , 3 years ago
Replying to Asif Saifuddin Auvi:
Replying to raydeal:
Replying to Pavel Druzhinin:
Replying to Mariusz Felisiak:
Thanks for this patch, however it's not ready for review. Please fix tests and
isort
.
Hello, it seems I need help to fix this error. I'm not sure code from previous contributor is correct and I don't fully understand the subject of this bug (m2m relations).
What is better to do: remove labeleasy pickings
and try another bug or someone could help me with fixing this bug?
P.S. Please, don't think for nagging. I really don't want to drop the ticket silently and want to continue to contribute at the same time. Thanks in advance with your advice how best to proceed.
Would you like to carry on with the ticket? I can help you. After reading comments and code my understanding is to make FK reverse working the same way as m2m when id=None, what is done in code, but message is a bit different, it would be good to have similar message in both cases. It will require amend m2m tests as well after update with current django version.
you should take over the ticket and create a new PR as it seems he is not able to fix the issue
I agree with this proposal, you could change an assignee. Thanks!
follow-up: 44 comment:43 by , 3 years ago
raydeal if you are currently unable to work on this ticket then you can assign it to me.
comment:44 by , 3 years ago
Replying to Ayush Joshi:
raydeal if you are currently unable to work on this ticket then you can assign it to me.
I am working on the ticket
comment:45 by , 3 years ago
Patch needs improvement: | unset |
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comment:46 by , 3 years ago
Needs tests: | set |
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Patch needs improvement: | set |
follow-up: 49 comment:47 by , 3 years ago
My patch is implemented using different approach than previous. It changes behaviour of FK to be the same as M2M.
I went through discussion in this and #17541 ticket and PR for them and analysed examples.
This information in the ticket
" There is a (slightly stale) patch for #17541 which makes fk fields and m2m fields work consistently."
might be true 9 years ago, but now it is not consistent with M2M.
I have also tested previous patch (https://github.com/django/django/pull/13784) locally.
I couldn't find correct rules because M2M worked as always, only changed behaviour of FK. When object is not saved it raises ValueError, when saved but related value is None returns <QuerySet []>, which is not consistent for me. Why?
Base on doubt from #17541 https://code.djangoproject.com/ticket/17541#comment:8 I asked myself: What is the difference between not saved object with id (pk) refrerenced from other object and saved object with field containing None value referenced by other object, through FK both, from the relation point of view?
There is no difference - both of them have None value and making related query in both cases doesn't make sens.
So finally I came to the conclusion that what M2M is doing is correct - in both cases it raises error - and it meet some of The Zen of Python rules, I think.
comment:48 by , 3 years ago
Needs tests: | unset |
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Patch needs improvement: | unset |
follow-up: 50 comment:49 by , 3 years ago
Patch needs improvement: | set |
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Replying to raydeal:
I couldn't find correct rules because M2M worked as always, only changed behaviour of FK. When object is not saved it raises ValueError, when saved but related value is None returns <QuerySet []>, which is not consistent for me. Why?
Base on doubt from #17541 https://code.djangoproject.com/ticket/17541#comment:8 I asked myself: What is the difference between not saved object with id (pk) refrerenced from other object and saved object with field containing None value referenced by other object, through FK both, from the relation point of view?
There is no difference - both of them have None value and making related query in both cases doesn't make sens.
There is a huge difference for me. It's doesn't matter which field is pointed by m2m field, that's not crucial. The most important thing is that Foo()
doesn't exist in the database so any related query makes no sense, and IMO we should raise an exception only in this particular case. You can start a discussion on DevelopersMailingList if you don't agree, where you'll reach a wider audience and see what other think.
Moreover proposed change is not a part of the ticket description, it's backward incompatible, and was not accepted per se.
comment:50 by , 3 years ago
Replying to Mariusz Felisiak:
Right, now I better understand what is expected solution. I am going to review again previous patch and change this one.
comment:51 by , 3 years ago
Patch needs improvement: | unset |
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comment:52 by , 3 years ago
Patch needs improvement: | set |
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comment:53 by , 3 years ago
Patch needs improvement: | unset |
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comment:54 by , 3 years ago
Triage Stage: | Accepted → Ready for checkin |
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comment:56 by , 3 years ago
Cc: | added |
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Ran across this while bringing Wagtail up to date with Django main. I appreciate that accessing a reverse-FK relation on an unsaved instance is undefined behaviour, so this isn't technically a breaking change - however, I suspect there could be a lot of code in the wild that's doing this unwittingly, in which case this change might be more disruptive than expected.
For example, I had to make this fix: https://github.com/wagtail/wagtail/pull/8028/commits/9e19bf63dbb698a4f57b131b6f8ae92bab7c2606 - where I'd implemented something approximating an InlineFormSet, but neglected to make a special case for the 'create' view, and so it ends up querying an unsaved instance to arrive at the initial empty formset. I imagine this will be a fairly common gotcha in user code.
(for the record, Wagtail also does much more esoteric things with faking relations on in-memory objects to support previews and saving drafts, but I'm willing to take the hit for that :-) )
Should we consider making this a deprecation warning for now, and adding the strict validation in Django 5.x instead? (Admittedly, this isn't the sort of thing you can write a startup check for, so it may be that a deprecation warning would only help for apps that have good test coverage anyhow.)
comment:57 by , 3 years ago
Should we consider making this a deprecation warning for now, and adding the strict validation in Django 5.x instead? (Admittedly, this isn't the sort of thing you can write a startup check for, so it may be that a deprecation warning would only help for apps that have good test coverage anyhow.)
IMO deprecation is unnecessary, we raise an exception so it's not something can be easily missed. Also, this is undocumented behavior and as far as I'm aware it should be quite rare. If you think deprecation is necessary, please first start a discussion on the DevelopersMailingList, where you'll reach a wider audience and see what other think.
comment:58 by , 14 months ago
I work on a large Django codebase and while upgrading it from version 3.2 to 4.2 a lot of code broke due to this change, where code that was once returning an empty queryset, now raises a ValueError
. In my opinion, this change should be fully reverted. The partial update from #33952 is not enough. I'm pretty sure I'm not alone in this situation. One of Django's core strengths has always been its stability and breaking user code without even a deprecation warning should be avoided at all costs.
comment:59 by , 10 months ago
I agree with the above. Why does something like this (where self
is an unsaved User
instance)
Book.objects.filter(author=self).exists()
display a proper RemovedInDjango50Warning
, while equivalent
self.books.exists()
suddenly crashes? This is totally unexpected.
Reviewing my own ticket as "DDN".