var params= {sublistId: 'item', line: line_num};
params.fieldId = 'inventorydetail';
params.line =0;
var curRecord = po_record.getSublistSubrecord(params);
params.sublistId = 'inventoryassignment';
params.fieldId = 'inventorynumbers';
var currentNumbers = curRecord.getValue(params);
params.value = [serial#];
curRecord.setValue(params);
var id = po_record.save({
enableSourcing: true,
ignoreMandatoryFields: true
});