Apex Trigger: Prevent Closure of Work Orders with Open Line Items

The following trigger prevents users from closing a work order unless all its line items have been closed. It’s a good way to ensure that all scheduled tasks are completed. The accompanying unit test scans your code for issues.

To define a work order trigger in your org:

From Setup, enter Work Orders in the Quick Find box, then select Triggers under Work Orders.
Click New.
Copy the trigger text and paste it into the text field.
Click Save.

1trigger ValidateWorkOrderLineItem on WorkOrder (before update) {
2    for(WorkOrder w : Trigger.New) {
3        if(w.Status =='Closed'){
4            List<WorkOrderLineItem> woLineItemList = [Select wo.Status
5                           From WorkOrderLineItem wo where wo.WorkOrderId=:w.Id];
6             if(woLineItemList.isEmpty()==false){
7                 for(WorkOrderLineItem woLineItem : woLineItemList){
8                     if(woLineItem.Status != 'Closed'){
9                          w.addError('You cannot close a work order until all of its line items are closed.');
10                     }
11                 }
12             }
13        }
14    } 
15}

Field Service Lightning Developer Guide

Apex Trigger: Prevent Closure of Work Orders with Open Line Items