Trigger causes system null pointer exception

Question

I'm having problem with my trigger.Please help me to resolve this system null pointer exception

trigger theatreupdate on Transaction__c (before insert) 
{

    for(Transaction__c t:Trigger.new)
    {

        t.TheaterT__r.Number_Of_Seats_Sold__c=0;
    }
//TheaterT__r-parent of Transaction__c


}

Answer 1

You cannot update parent fields directly. Here is how to do it

trigger theatreupdate on Transaction__c (before insert) 
{
    Set<String> parentIds = new Set<String>();
    for(Transaction__c t:Trigger.new)
    {
       parentIDs.add(t.TheaterT__c);
    }

    List<Theater__c> theatres = [select Number_Of_Seats_Sold__c from Theater__c where id = :parentIds];

    for (TheaterT__c th : theatres_ }
       th.Number_Of_Seats_Sold__c=0;
    }

    update theatres;

}

Answer 2

You can not update parent fields directly in trigger. You have to create new instance and then do update. Only current object level fields can be updated.