Accessing a deleted model field attribute triggers a query to reload the field rather than raising AttributeError (behavior change in 1.10)

[Julien Hartmann]

This is a consequence of the implementation of #26207 in PR ​6118

Now, all fields have a DeferredAttribute descriptor. This has the consequence that this will trigger a model reload:

obj = MyModel.objects.get(pk=1)
del obj.title
obj.title

Up to version 1.9 (included), this code would raise an AttributeError. Now it silently triggers a query to reload the field from the database.

Is this intended? If so, it should probably be added to the release notes.

[Tim Graham]

The description of that PR gives an example of the behavior you describe:

One can also do the following after the patch:

obj = MyModel.objects.first()
del obj.field
obj.field  # <- Loads the field from DB

so I'd say that yes, that's what Anssi intended. Whether or not there's justification to change the behavior is another question, although the old behavior seems untested and undocumented so backwards-compatibility likely isn't extremely critical.

Could you explain your use case? Are there any ways to achieve it after the change?

[Julien Hartmann]

I don't have a use case.

I maintain a third-party package (django-hvad), that adds transparent model translation functionalities. To ensure behavior of hvad-enabled models is consistent with vanilla django models, I have extensive test cases in place. That's how I detected this change.

I went through the release notes and found nothing, so I decided to open a ticket. Documenting the change in behavior in the release notes is perfectly fine with me. :)

[Tim Graham]

​PR

[Julien Hartmann]

Great! So from the test case I understand it is guaranteed that only the accessed field is reloaded.

I'll update my own test case to check this as well. Support for Django v1.10 will be complete soon, thanks for being so reactive.

[Tim Graham <timograham@…>]

In 20d1cb33:

Fixed #26787 -- Documented deleting and reloading of model instance fields.

Thanks Julien Hartmann for the report.

[Tim Graham <timograham@…>]

In 45a6507:

[1.10.x] Fixed #26787 -- Documented deleting and reloading of model instance fields.

Thanks Julien Hartmann for the report.

Backport of 20d1cb33c2ec1242e16c49deb88e8c70517b2d1a from master