/* get the id of the customer. */
int
Transaction::getCustomerId (const int tran_id) {
    /* declare a sql query object. */
    QSqlQuery query;

    /* prepare a sql query with place holders. */
    query.prepare("SELECT cust.id FROM customer AS cust INNER JOIN transacts AS tran ON cust.id = tran.cust_id WHERE tran.id = :tran_id");

    /* bind values to the query placeholders. */
    query.bindValue(":tran_id", tran_id);

    /* execute the query. */
    query.exec();

    /* the customer's id (assume not found). */
    int cust_id = -1;

    /* try to get the first record. */
    if (query.next()) {
        /* get the id of the customer. */
        cust_id = query.value(0).toInt();
    }

    /* return the id of the customer. */
    return cust_id;
}