Trigger On Opportunity Stage based on Opplineitem

Question

I have custom field Invoice Date in OpportunityLineitem when this is left blank it should show error using trigger when trying to update Opportunity with Opportunity stagename as closed won.

I tried the following way:

trigger OpportunityStageWon on Opportunity(before insert, before update) {
    set<Id> Oid = new set<id>();
    for (Opportunity o: Trigger.new) {
        Oid.add(o.id);
    }
    List<OpportunityLineItem> OLI = [Select id, name, Invoice_Date1__c from OpportunityLineItem where Opportunityid =: Oid];
    for (Opportunity o: Trigger.New) {
        for(OpportunityLineItem ol : OLI){
            if (o.StageName == 'Closed Won' && ol.Invoice_Date1__c == null) {
                o.StageName.addError('Your custom error message');
            }
        }
    }
}

Answer 1

Try Below Code

trigger OpportunityStageWon on Opportunity(before insert, before update) {
    set<Id> Oid = new set<id>();
    for (Opportunity o: Trigger.new) {
        Oid.add(o.id);
    }
    List<OpportunityLineItem> OLI = [Select id, name, Invoice_Date1__c from OpportunityLineItem where Opportunityid =: Oid];
    for (Opportunity o: Trigger.New) {
        for(OpportunityLineItem ol : OLI){
            if (o.StageName == 'Closed Won' && ol.Invoice_Date1__c == null) {
                o.StageName.addError('Your custom error message');
            }
        }
    }
}

UPDATE:

You can refer below link for Trigger.New and Trigger.Old Difference

https://developer.salesforce.com/forums/?id=906F000000098JAIAY

Hope this might helps...

Answer 2

So I guess you want to show en error when opportunity status gets changed to Closed Won and Invoice_Date1__c is empty?

Let's analyse your code line per line:

set < Id > id = new set < id > ();

You create a set of ids and call id just id. Not the best practice in my opinion, but whatever, it will compile and even work

OpportunityLineItem OL = [Select id, name, Invoice_Date1__c from OpportunityLineItem where Opportunityid =: id];

I guess your execution fails here with System.QueryException: List has no rows for assignment to SObject? You are trying to query a single OpportunityLineItem where the reference to Opportunity is an empty set. You should do this query after the next loop(and use IN operator instead of = and use a List instead of a single OLI(or add LIMIT 1 to your query)):

for (Opportunity o: Trigger.new) {
    id.add(o.id);
}

Make sure to do this before querying your OLIs.

for (Opportunity o: Trigger.old) {
    if (o.StageName == 'Closed Won' && ol.Invoice_Date1__c == null) {
        o.StageName.addError('Your custom error message');
    }
}

If you loop through Trigger.old, o.StageName will be the stageName before the update, so if you only change it from Open to Closed Won, this will never be in the check.

You should compare old to new to notice the change of the field. And you should also use list of OLIs, because there can be multiple OLIs on one Opportunity.