Return the field object with the given name (for the class, the fields
        are in the "_redis_attr_%s" form)

def get_class_field(cls, field_name):
        """
        Return the field object with the given name (for the class, the fields
        are in the "_redis_attr_%s" form)
        """
        if not cls.has_field(field_name):
            raise AttributeError('"%s" is not a field for the model "%s"' %
                                 (field_name, cls.__name__))

        field = getattr(cls, '_redis_attr_%s' % field_name)

        return field