ModelSerializer: Using a class that does not inherit from ChoiceField strips allow_blank

[amhn]

While trying to improve schema generation for netbox, I stumbled on something:
I wanted to change the default serializer for fields with choices, so in a subclass of ModelSerializer I set serializer_choice_field to netbox.api.fields.ChoiceField which is not a subclass of rest_framework.fields.ChoiceField.

Everything looked fine except for fields with allow_blank=True. This was omitted from the schema.

I think this is because of the following check:

django-rest-framework/rest_framework/serializers.py

Lines 1296 to 1298 in e2a4559

	if not issubclass(field_class, CharField) and not issubclass(field_class, ChoiceField):
	# `allow_blank` is only valid for textual fields.
	field_kwargs.pop('allow_blank', None)

I think either this check should be changed to use self.serializer_choice_field instead of ChoiceField or moved before Line 1272:

django-rest-framework/rest_framework/serializers.py

Lines 1272 to 1275 in e2a4559

	if 'choices' in field_kwargs:
	# Fields with choices get coerced into `ChoiceField`
	# instead of using their regular typed field.
	field_class = self.serializer_choice_field

Did I miss something?

[auvipy]

what issue is it causing actually?

[amhn]

Everything looked fine except for fields with allow_blank=True. This was omitted from the schema.

And (I assume) the field does not accept blank values afterwards (not tested).

[auvipy]

can you start with a failing test?

[amhn]

Sorry. The result was not, what I had in mind and the changes are already reverted. Just thought to give you a heads up for the bug.

With the original ChoiceField I ended up with ChoiceField(allow_blank=True, [...]) as serializer for a CharField with allow_blank=True and choices set, with the ChoiceField from netbox it was ChoiceField([...]) for the same model field.