ChoiceField should support a child just like the ListField

[michal-klimek]

Dear people,

I have a custom Django field LanguageField that works with a data class Language which itself is a container for valid BCP 47 language tags. The Language has other properties specific to the language tag domain. I have also a LanguageField that subclasses serializers.Field and implements all required serialization/deserialization logic. As the django field indirectly subclasses the models.CharField I can use choices argument, and thus I expect DRF to handle it accordingly. I use ModelSerializer.serializer_field_mapping to let DRF know how to deal with my custom LanguageField automatically without the need of declaring it explicitly in my serializers.

However it does not work along with the choices option. This is because the ChoiceField throws away the information about the source model field and it's possible requirements. If the problem was only in the ChoiceField I wouldn't bother, because I could just replace the ChoiceField class by my own one. But the real problem lies in the code that transforms the origin field into the ChoiceField and strips away information not supported by the ChoiceField.

The following link points to a place with the issue code:

django-rest-framework/rest_framework/serializers.py

Line 1238 in 5bea22f

if 'choices' in field_kwargs:

I suggest to refactor the ChoiceField in a similar way the ListField works, by adding a child feature. This way ChoiceField may honor all child-specific requirements and may act as a proxy that uses serialization/deserialization from the child field.

Currently the whole serialization fails, throwing an exception, because a Language object is directly passed to the JSON serializer which simply can not handle it.