"Assigned to" field to be read only on reassignment to another group

[Naveen10]

I want to make the "Assigned to" read-only for assignments only. After the tickets have been assigned (And saved), users should be able to assign them to a group member. after reassignment to another group the Assigned to" field should be read-only. I have written the UI policy but it's not working when I assign an assignment group after assigning it to the person it shows read-only but I want to read only when I re-assign it to another group. please advise

[Mark Manders]

You are reassigning to another group, and after that, the assigned to can't be changed? Isn't that weird? It's not supposed to be at the servicedesk, so it's assigned to the application team, but assigned to is still the servicedesk agent? How is someone from the application team going to take ownership of the ticket? 

Can you elaborate on the process?

[Naveen10]

My scenario is when I am assigning to an assignment group after that assign to some group member after saving the form again if I want to reassign another group that time assigned to the field should be read only that should work on 1st assignment group people only those people only should be able to work on that particular incident.

assignment group(filled)-->assigned to field(filled)-->save the form-->again I am changing the assignment group(filled)-->that time assigned to field should be read-only

[Naveen10]

I have written on change client script but it's not working can you please help anyone

function onChange(control, oldValue, newValue, isLoading, isTemplate) {

   if (isLoading || newValue === '') {

      return;

   }

 if(control == 'assignment_group'){

    var assignmentGroup = g_form.getValue('assignment_group');

    var previousAssignmentGroup = g_form.getPreviousValue('assignment_group');

    if(assignmentGroup != previousAssignmentGroup){

        // var assignedTo = g_form.getValue('assigned_to');

        // if(assignedTo){

        g_form.setReadOnly('assigned_to', true);

        alert('naveen');

        //}

 }

 }

}

[Mark Manders]

Client side, you can only have it work one time: when you change it. Next time the record is opened, there isn't a 'previous' assignment group anymore. 

I still don't see the logic behind the requirement. Normally there are reference qualifiers making sure that you can only 'assign_to' to members of the 'assignment_group' or even the other way around. Are you sure you don't have that kind of logic in place, because that can intervene with your logic. 

Can you explain why the reassigning is done on group, but not on user? In processes I have helped setup, the assigning to another group is normally done, because you need someone else (who is member of that group) to pick it up (lack of time/knowledge/skills on the first assignment). Assigning to another group, but keeping it assigned to the same person just sounds like a waste of time.

If you are absolutely set on doing it like this, you could create a new 'assignment group' field that you hide from the form. On first assignment you set that field to the current assignment group and you let a UI policy make 'assigned to' read only, when the 'assignment group' and your new field aren't the same anymore.