Allow ModelAdmin.radio_fields to work with NullBooleanField

[Jerome Leclanche]

How to reproduce:

• Create a model with a NullBooleanField "foo"
• Create and register a corresponding ModelAdmin instance for the model
• Add radio_fields = {"foo": admin.HORIZONTAL} to the model admin

It will be rejected with a SystemCheckError:

<class 'djstripe.admin.EventAdmin'>: (admin.E023) The value of 'radio_fields' refers to 'foo', which is not an instance of ForeignKey, and does not have a 'choices' definition.

This is very unintuitive. I'm unsure why even it *needs* a choices definition seeing as it has no problem rendering a <select> widget.

Relevant links:

• ​https://stackoverflow.com/questions/11990852/django-display-nullbooleanfield-as-radio-and-default-to-none
• ​https://groups.google.com/forum/#!topic/django-users/HFU-ub1kDMw
• https://code.djangoproject.com/ticket/23681

[Tim Graham]

It's not as simple as removing the system check. Changes in BaseModelAdmin.formfield_for_dbfield are also required to add support for NullBooleanField in radio_fields. Fixing #23130, which deprecates NullBooleanField in favor of allowing null=True on BooleanField, may help or even completely solve this issue.

[Musen]

Tim, do you know the status of #23130? It seems to be inactive for 3 years. Do people come to the conclusion to deprecates NullBooleanField?

Also, I can see 2 ways to finish the ticket: one to hook NullBooleanField and create a ChoieField with (('1', 'Unknow'), ('2', ‘Yes'), ('3', 'No') instead (when it is in radio_fields); another is to create a NullBooleanRadioSelect widget to handle this like the original NullBooleanSelect widget. I am not which one is better or there could be an even better way.

[Tim Graham]

Yes, I think we should try to deprecate NullBooleanField. I gave an update in ticket:23130#comment:20.

[Jacob Walls]

In d976c254fc76e5f04d81dfd9d142c58e933c9c92 the decision was made to document providing choices to forms.NullBooleanField in order to use Select or RadioSelect widgets. I believe that also resolves this issue, since the error message is now appropriate:

(admin.E023) The value of 'radio_fields' refers to 'foo', which is not an instance of ForeignKey, and does not have a 'choices' definition.

See discussion on tickets #23681 and #27445.