How to fire a trigger on formula field by updating?

Question

Here Amount_c is the formula field and Commission_Total__c is the parent object field where the sum needs to be updated while changing the amount in child object.but the trigger is not firing..please give me solution

trigger totalCommissionAmountTrigger on Commission__c (after insert, after update) 
{
    public Map <ID, List<Commission__c >> mapIdList = NEW   Map <ID, List <Commission__c >> ();
    public List <Commission__c > commissionList = NEW List<Commission__c >();
    public List <AVTRRT__Placement__c> placementList = NEW List <AVTRRT__Placement__c>();
    Set <ID> placementId = NEW Set<ID>();   
    if( (Trigger.isInsert) || (Trigger.isUpdate) )
    {
        System.debug('**************');
        for (Commission__c cms: Trigger.New)
        {
            if (cms.Placement__r.Id != null)
            {
                placementId.add(cms.Placement__r.Id);
                system.debug('-----'+placementId);
            }
        }      
        commissionList = [ SELECT 
                             Spread__c ,Commission__c, Amount__c, Placement__r.Id
                           FROM 
                            Commission__c 
                           WHERE    
                            Placement__r.Id IN:placementId];
        for (Commission__c commission:commissionList)
        {
            if (commission.Amount__c !=null)
            {

                mapIdList.put(commission.Placement__r.Id,commissionList);


            }
         }

        placementList = [SELECT Commission_Total__c FROM AVTRRT__Placement__c WHERE Id IN:placementId];
        List <AVTRRT__Placement__c> updatePlacementList = NEW List <AVTRRT__Placement__c>();
        Decimal total = 0;
        for(AVTRRT__Placement__c p:placementList)
        {


               for(Commission__c c:mapIdList.get(p.Id))
               {                     
                     System.debug('////////////'+c.Amount__c); 
                     if (c.Amount__c!=null) 
                     {

                         total += c.Amount__c;
                     }

               }
               p.Commission_Total__c  = total;                  
               system.debug('--------'+p.Commission_Total__c);
               updatePlacementList.add(p);
         }
        update updatePlacementList;

     }
}

Answer 1

You can have a rollup field and a workflow to update another amount field when rolled up amount is being changed. In this case you can have a trigger to kick in to do what you want it to do on a parent record.