Fetch sales person form customer master

how to fetch sales person form customer master to another document when select customer name normal fetch script not working for tables
cur_frm.add_fetch("customer", "sales_person", "sales_person");

Did you try to make the sales_person as read only and then fetch it by the option field like customer.sales_person, or if you are in version 11 it will be in the newly docfield which its name is Fetch From