@classmethod
def _combined_fields(cls, field_name):
    fields = []
    for clazz in cls.__mro__:
        fields.extend(getattr(clazz, field_name, []))
    return fields
@classmethod
def _combined_fields(cls, field_name):
    fields = []
    for clazz in cls.__mro__:
        fields.extend(getattr(clazz, field_name, []))
    return fields