Trigger Not Firing - When Child Updated, Update Field on Master

Question

Have two custom objects, Master and Child. Child has a lookup to Master.

Trigger is saving, but not firing when I update the Child:

trigger updateMasterName on Child_Custom__c (before insert, before update){
    list<Id> masIds = new list<Id>();
    list<Master_Custom__c> masters = new list<Master_Custom__c>();
    for(Child_Custom__c cc:trigger.new){
        masIds.add(cc.Master_Custom__r.id);
    }
    for(Master_Custom__c mas:[select Id from Master_Custom__c where Master_Custom__c.id IN :masIds]){
        for(Child_Custom__c ccc : trigger.new){
            if(ccc.Name == 'aaa'){
                ccc.Master_Custom__r.Name ='Wow this worked';
                masters.add(mas);
            }
        }
    }
    update masters;
}

Answer 1

Ok, so the problem is that you are setting a field through the relationship:

ccc.Master_Custom__r.Name = 'Wow this worked';

However, what you need to do is actually set the name field on the Master object itself: ie

mas.Name = 'Wow this worked';

I think this will fix it for you.

To further illustrate, you essentially have a bunch of lists in memory, one of children records, and one of master records. Since you are modifying a property of the child record and saving the master list, the change never appears.

Answer 2

Your logic to identify the master to update is a little wonky. You're not associating the master with its proper child. Check out my sample below.

trigger updateMasterName on Child_Custom__c (before insert, before update){
    list<Id> masIds = new list<Id>();
    list<Master_Custom__c> masters = new list<Master_Custom__c>();

    for(Child_Custom__c cc:trigger.new){
        masIds.add(cc.Master_Custom__c);
    }

    Map<Id, Master_Custom__c> masters = new Map<Id, Master_Custom__c>([select Id from Master_Custom__c where id IN :masIds]);

    for(Child_Custom__c ccc : trigger.new){            
        if(ccc.Name == 'aaa'){
            if (masters.containsKey(ccc.Master_Custom__c)) {
              Master_Custom__c mas = masters.get(ccc.Master_Custom__c);
              mas.Name ='Wow this worked';

              masters.add(mas);
            }
        }
    }

    update masters;
}