[Django] #35125: Parent model fields are ignored when preparing the update

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Jan 18, 2024, 9:24:47 AM1/18/24

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This might be an edge case but I'd like to report this functionality that
might be slightly broken, as someone coming from Django 3.X to Django 4.X.

According to
[https://docs.djangoproject.com/en/4.2/ref/models/fields/#django.db.models.DateField.auto_now
4.2 the docs], using `auto_now` in model fields might not result in actual
updates after the `save()` method is called.

However, the code has a piece for backward compatibility, where these
fields are indeed picked up:
https://github.com/django/django/blob/4.2.3/django/db/models/query.py#L959-962

This raises the first problem, which is the inconsistency between
documentation and the actual code. I appreciate the backward compatibility
though, it's a lifesaver for my specific use case :)

The other issue I am having is with a potentially unusual model scheme
where I have parent classes for some models. These are not abstract
classes, but sorts of aggregator classes:

{{{
class A(model.Model):
updated_at = models.DateTimeField(auto_now=True)
something = models.BooleanField(default=True)

# in another app
class B(A):
name = models.CharField()
}}}

Previously, when we did `B.objects.update_or_create(name="x",
defaults={"something": False})` it would automatically update `updated_at`
timestamp. In 4.2 (and above, apparently) this is no longer happening.
**However**, if `updated_at` was a field in the `B` class, `updated_at`
would be picked up for update with the same `update_or_create` call.
Notice I do not need to specify the `updated_at` field.

My workaround for now will be to override the save method of the parent
classes to include the `updated_at` field in the `update_fields` list.

[https://github.com/django/django/blob/4.2.3/django/db/models/query.py#L963
when line 963 of the query] is called it only uses the local model fields
to prepare the new `update_fields`. This drops any field coming from the
parent class.

----

I'd like to propose to use `self.model._meta._non_pk_concrete_field_names`
instead of `self.model_meta.local_concrete_fields` in the above line of
code, or any other attribute that contains the parent fields that its
better suited. If this is accepted as an issue, I can create a patch and
submit it for review/approval.

Django version: 4.2.3
Python: 3.11.1

This is the first time I am contributing to Django - I apologise if there
is a better way to report issues or if there are any templates to follow.

Feel free to ask any question or ask for what is not clear in this
description.

Thanks in advance.

--
Ticket URL: <https://code.djangoproject.com/ticket/35125>
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Django

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Jan 18, 2024, 8:41:21 PM1/18/24

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#35125: Parent model fields are ignored when preparing the update_fields
-------------------------------------+-------------------------------------

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Natalia Bidart):

* keywords: models, update_or_create => update_or_create
* status: new => closed
* resolution: => needsinfo
* type: Uncategorized => New feature

Comment:

Hello Gustavo! Thanks for your report.

If I understand your second part of the report correctly, your issue is
similar to the one described in this forum post:
https://forum.djangoproject.com/t/update-or-create-behavior/25944/ (while
the first post is about a new feature request, I think the underlying
issue is the same and you may benefit from the posted answers). If yes,
could you please add your proposal/concern to that topic? Or you could
also create a new topic to present your idea, since you'll reach a wider
audience and likely get extra feedback in the forum than here.

For the first part of the report, could you please cite the exact part of
the linked doc that you find confusing?

I'll close this as needsinfo for now, but feel free to comment back in the
ticket with further information so we can re-triage. Thanks again!

--
Ticket URL: <https://code.djangoproject.com/ticket/35125#comment:1>

Django

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Jan 19, 2024, 5:00:22 AM1/19/24

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#35125: Parent model fields are ignored when preparing the update_fields
-------------------------------------+-------------------------------------
Reporter: Gustavo Silva | Owner: nobody
Type: New feature | Status: closed
Component: Database layer | Version: 4.2
(models, ORM) |
Severity: Normal | Resolution: needsinfo
Keywords: update_or_create | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Gustavo Silva):

Hi Natalia, thank you very much for your reply!

Replying to [comment:1 Natalia Bidart]:

> Hello Gustavo! Thanks for your report.
>
> If I understand your second part of the report correctly, your issue is
similar to the one described in this forum post:
https://forum.djangoproject.com/t/update-or-create-behavior/25944/ (while
the first post is about a new feature request, I think the underlying
issue is the same and you may benefit from the posted answers). If yes,
could you please add your proposal/concern to that topic? Or you could
also create a new topic to present your idea, since you'll reach a wider
audience and likely get extra feedback in the forum than here.

Thank you for pointing me in that direction. I will take a look to that
post and will try to see if it is the same problem - and being so I can
propose a fix there, sure.

Replying to [comment:1 Natalia Bidart]:

> For the first part of the report, could you please cite the exact part
of the linked doc that you find confusing?

I highlighted the bits that was confusing but perhaps they are only
confusing when considering the release notes and/or the queryset
reference. So, the model field reference says this about `auto_now`:

> The field is only automatically updated when calling Model.save(). The
field isn’t updated when making updates to other fields in other ways such
as QuerySet.update(), though you can specify a custom value for the field
in an update like that.

https://docs.djangoproject.com/en/4.2/ref/models/fields/#django.db.models.DateField.auto_now

But the release notes mention this, about the fields that are picked up
for updates: https://docs.djangoproject.com/en/4.2/releases/4.2/#setting-
update-fields-in-model-save-may-now-be-required

The docs for phttps://docs.djangoproject.com/en/5.0/ref/models/querysets
/#update-or-create/ the `update_or_create` method also mention that after
4.2 only some fields are selected for update.

All of this in conjunction, led me to believe auto_now fields do not get
picked up for updates after 4.2 but maybe it's only my interpretation, I
don't know.

--
Ticket URL: <https://code.djangoproject.com/ticket/35125#comment:2>

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