Update all trigger records with new field value

Question

I'm trying to update OwnerId on the a List<Case> once the fields are updated on a case object, only first case is getting updated but not all cases. I need to update all cases with same OwnerId.

trigger caseAssignment on Case (after insert, after update) {

    set<id> ownerId = new Set<Id>();
    set<id> customerId = new set<Id>();

    for(Case caseobj : trigger.new) {
        ownerId.add(caseobj.OwnerId); 
        customerId.add(caseobj.AccountId);
    }

    User user = [Select id, FirstName, LastName from user where Id IN  :ownerId];

    for(Case cas : [Select Id, OwnerId, First_Name__c, CaseNumber, AccountId From Case where AccountId IN: customerId]) { 
        cas.OwnerId = user.Id;
    }   
}

Answer 1

As as said in my comment, the problem is that you are assigning all the cases found to the last user of your for loop.

Also, you're not calling an update on the cases list.

This should work:

trigger caseAssignment on Case (after insert, after update) {
    Map<Id, Id> ownerIdByAccountId = new Map<Id, Id>();
    Set<Id> currentCasesIds = new Set<Id>();
    for(Case caseobj : trigger.new){
        Boolean caseObjChanged = Trigger.isUpdate && (Trigger.oldMap.get(caseobj.Id).AccountId != caseobj.AccountId || Trigger.oldMap.get(caseobj.Id).OwnerId != caseobj.OwnerId);

        if(Trigger.isInsert || caseObjChanged){
            ownerIdByAccountId.put(caseObj.AccountId, caseObjOwnerId);
            currentCasesIds.add(caseobj.Id);
        }
    }

    List<Case> casesToUpdate = new List<Case();
    for(Case cas : [Select Id, OwnerId, AccountId From Case where AccountId IN: ownerIdByAccountId.keySet() AND Id NOT IN :currentCasesIds]){       
        Id newOwnerId = ownerIdByAccountId.get(cas.AccountId);
        if(cas.OwnerId != newOwnerId){
            cas.OwnerId = newOwnerId;

            casesToUpdate.add(cas);
        }
    }

    update casesToUpdate;
}

The first loop is adding only the cases with the accountid changed or the new cases.

Thanks to a Map, you are able to link the cases account id to an owner id to assign it right later.

Finally, we call an update on the cases that changed.