public function getNzMmsId()
    {
        // If initialized from an SRU record, we need to fetch the
        // remaining parts of the Bib record.
        $this->init();

        $nz_mms_id = $this->data->text("linked_record_id[@type='NZ']");
        if (!$nz_mms_id) {
            throw new NoLinkedNetworkZoneRecordException("Record $this->mms_id is not linked to a network zone record.");
        }

        return $nz_mms_id;
    }