@staticmethod
def get_warning_for_deferred_fields(instance, using=None, fields=None):
    """Accessing deferred fields will call refresh_from_db for that field."""
    if fields is None:
        return None
    deferred_fields = set(fields).intersection(instance.get_deferred_fields())
    if deferred_fields:
        return 'Accessing deferred field(s) {}'.format(', '.join(
            deferred_fields))
    return None
