beforeUpdate System.FinalException: Record is read-only

Question

I'm making a Trigger that when adding a product to the Opportunities object when updating the Quantity if placed below the original value an error will appear, but when increasing the date field it would have to be updated automatically to the current day

but it's giving an error, what could I be doing or what am I doing wrong?

OpportunityLineItemTrigger: execution of BeforeUpdate caused by: System.FinalException: Record is read-only Class.OpportunityLineItemHelper.checkChangeQuantity: line 11, column 1

public static void checkChangeQuantity(Map<Id, OpportunityLineItem> oldMap, Map<Id, OpportunityLineItem> newMap) {
    for (OpportunityLineItem newItem : newMap.values()) {
        OpportunityLineItem oldItem = oldMap.get(newItem.Id);

        if (newItem.Quantity > oldItem.Quantity) {
            Trigger.newMap.get(newItem.Id).addError(System.Label.IncreaseQuantityError);
        }
        else if (newItem.Quantity < oldItem.Quantity) {
            newItem.ServiceDate = Datetime.now().date();
        }
    }
}

public static List<Opportunity> checkItemChange(Map<Id, OpportunityLineItem> oldMap, Map<Id, OpportunityLineItem> newMap) {
    Set<Id> opportunityIdSet = new Set<Id>();

    for (OpportunityLineItem newItem : newMap.values()) {
        OpportunityLineItem oldItem = oldMap.get(newItem.Id);

        if ((newItem.Quantity != oldItem.Quantity) || (newItem.UnitPrice != oldItem.UnitPrice)) {
            opportunityIdSet.add(newItem.opportunityId);
        }
    }

    List<Opportunity> opportunityList = [
        SELECT Id, StageName
        FROM Opportunity
        WHERE Id IN: opportunityIdSet
        AND StageName <> 'Prospecting'
    ];

    for (Opportunity opp : opportunityList) {
        opp.StageName = 'Prospecting';
    }
    return opportunityList;
}

Answer 1

The most likely cause of this error is that you're passing in Trigger.oldMap and Trigger.newMap in the incorrect order. Trying to update a record in Trigger.oldMap results in this exception for any type of trigger. You can only call addError on those records, and only in a delete (either before or after) trigger context. Check the OpportunityLineItemTrigger trigger where you're calling this method from and verify the order of the parameters are correct.