Document that Field.choices are enforced by model validation

[Evgeny Arshinov]

This is basically a sequel of #6967.

For some reason, only form validation was considered in the original ticket, but data validation is not limited to that, but also includes data integrity checks running when one calls model_instance.save().

As a developer, I would expect model_instance.save() to validate the choices field against the list of possible choices, not least because the list is defined at the model level, participates in db migrations etc., so it should take effect on model instances.

I haven't been able to find any documentation or existing bug reports that clearly state that ​choices argument only affects the corresponding model form field presentation and does not ensure data integrity.

In summary:

1. I would like to know the rationale for ignoring choices during data integrity checks, in case I am missing something.
2. Please consider providing an option to turn data integrity check on.
3. If the current behavior is left intact, it should be explicitly documented in the official documentation.

[Tim Graham]

Did you mean to title the ticket "Model.save() doesn't validate CHOICES"?

This is ​documented:

Note, however, that like Model.full_clean(), a model’s clean() method is not invoked when you call your model’s save() method.

This is because of backwards compatibility (and probably performance considerations).

There are ways to ​change the behavior in your project.

[Evgeny Arshinov]

Replying to Tim Graham:

Did you mean to title the ticket "Model.save() doesn't validate CHOICES"?

Yes. Sorry for the incorrect title, I fixed that.

This is ​documented:

Note, however, that like Model.full_clean(), a model’s clean() method is not invoked when you call your model’s save() method.

OK, I think I did not pay enough attention to the point that saving and validation are two different steps, as documented in the ​model instance reference, and fields are generally validated during clean() except for explicitly documented cases like ​unique (which is validated in save()).

However, it seems that many people stumble upon this issue. Maybe an explicit note could be added to the documentation for the choices parameter?

This is because of backwards compatibility (and probably performance considerations).

There are ways to ​change the behavior in your project.

OK, thank you for the direct link. We will consider using this approach in our project.

[Tim Graham]

​PR

[GitHub <noreply@…>]

In 8b1d361f:

Fixed #29549 -- Doc'd that Field.choices are enforced by model validation.

[Tim Graham <timograham@…>]

In 50dc9fad:

[2.1.x] Fixed #29549 -- Doc'd that Field.choices are enforced by model validation.

Backport of 8b1d361f28c80cb0fa84a3714d711174bd25cdfa from master