System.NullPointerException: Attempt to de-reference a null object - trying to prevent dupes with trigger

Question

I created a trigger that work very except that I got this error when I try to use addError :

MyTestTrigger: execution of BeforeInsert caused by: System.NullPointerException: Attempt to de-reference a null object Trigger.MyTestTrigger: line 19, column 1

Here is my trigger:

trigger CheckDupesTrigger on Pret__c (before insert) {

    Set<Id> livreNew = new Set<Id>{};
    Set<Id> contactNew = new Set<Id>{};
    for(Pret__c p: Trigger.new){

        livreNew.add(p.Livre__c);
        contactNew.add(p.Contact__c);
    }


    List<Pret__c> relatedPrets = [SELECT Id, Contact__c, Livre__c FROM Pret__c
        WHERE Livre__c IN :livreNew AND Contact__c IN :contactNew];
    system.debug('relatedPrets '+relatedPrets);

    if(!relatedPrets.isEmpty()){
        for(Pret__c p: relatedPrets){
            system.debug('Trigger p.Id '+p.Id);
            Trigger.newMap.get(p.Id).addError('Error - dupe item'); //I've tried with oldMap, still failed
        }
    }
}

Pret__c is a junction object between Livre__c and Contact__c

---

Edit:

I saw that p.Id has a value in the debug console.

Where I'm wrong ?

Answer 1

Before insert doesn't have Trigger.newMap as these records not yet inserted into DB, so we don't have Id as key in the Trigger.newMap.

trigger CheckDupesTrigger on Pret__c (before insert) {

  Set<Id> livreNew = new Set<Id>{};
  Set<Id> contactNew = new Set<Id>{};
  for(Pret__c p: Trigger.new){
    livreNew.add(p.Livre__c);
    contactNew.add(p.Contact__c);
  }


  List<Pret__c> relatedPrets = [SELECT Id, Contact__c, Livre__c FROM Pret__c WHERE Livre__c IN :livreNew AND Contact__c IN :contactNew];
  system.debug('relatedPrets '+relatedPrets);

  if(!relatedPrets.isEmpty()){
    // As relatedPrets is not empty it means having some duplicate records so adding error to the list.
    for(Pret__c p: Trigger.new){
      p.addError('Error - dupe item');
    }
  }
}