una/plugins/chargebee/ChargeBee/Models/CustomerReferralUrl.php

<?php

class ChargeBee_CustomerReferralUrl extends ChargeBee_Model
{
  protected $allowed = array('external_customer_id', 'referral_sharing_url', 'created_at', 'updated_at', 'referral_campaign_id', 'referral_account_id', 'referral_external_campaign_id', 'referral_system');

}

?>