AP polid minus 'polid' prefix if polid else ``None``.

def get_polid(self, obj):
        """AP polid minus 'polid' prefix if polid else ``None``."""
        ap_id = obj.candidate_election.candidate.ap_candidate_id
        if 'polid-' in ap_id:
            return ap_id.replace('polid-', '')
        return None