Validation Errors and null (for empty forms!) are not properly rendered

[s4ke]

Summary

Errors and default values for JSON fields are not rendered nicely in the browsable API when using nested serializers.

Original discussion from #6243

Detail

Right now, the browsable API has the following behaviour for json fields for nested bound fields:

1. You put in a broken/non-json payload into the JSON field in the browsable api, e.g. {'broken': json}. It quotes the input again to produce "{'broken': json}" in the browsable api on the error page.
2. You put in a null json value that makes validation fail (json field is not nullable). It quotes the input to "null" on the error page.
3. The default value is set to "" on an empty form, which is semantically different to an empty field. I think it would be better to default to the string "null".

While not a big issue, this is a nuissance when testing out the browsable API and you mistyped before sending a request as you have to manually fix the error in your payload as well as the extra quotes around it that were introduced by drf.

Without my snippet:

With my snippet:

Possible Fix

This could be the fix:

class PayloadNestedField(NestedBoundField):

    def as_form_field(self) -> Any:
        values = {}
        for key, value in self.value.items():
            if isinstance(value, (list, dict)):
                values[key] = value
            else:
                field = self.fields[key]
                error = self.errors.get(key) if isinstance(self.errors, dict) else None  # type: ignore
                if getattr(field, '_is_jsonfield', False) and error is not None or value is None:
                    if value is None:
                        values[key] = None
                    else:
                        class JSONString(str):
                            def __new__(cls, value):
                                ret = str.__new__(cls, value)
                                ret.is_json_string = True
                                return ret
                        values[key] = JSONString(value)
                else:
                    values[key] = '' if (value is None or value is False) else force_str(value)  # type: ignore
        return self.__class__(self._field, values, self.errors, self._prefix)  # type: ignore

[s4ke]

@tomchristie Thanks for moving it here. I did not read that message in the Issue template properly.