Inserting a child record using a trigger based on an external ID

Question

I have a trigger that inserts a new child record (Training Session) below Training Module using a third object (Scheduler). Training Module contains an external ID called Training_Module_ID, but when I enter that into Scheduler and save it gives me the following error:

Error: Invalid Data. Review all error messages below to correct your data. Apex trigger createTrainingSession caused an unexpected exception, contact your administrator: createTrainingSession: execution of AfterInsert caused by: System.StringException: Invalid id: Test: Trigger.createTrainingSession: line 7, column 1

However, if I type in the 15 character Record ID, then the Training Session is created as intended. How can I make this work using the external ID?

trigger createTrainingSession on Scheduler__C(after insert) {
    List < Training_Session__c > TStoinsert = new List < Training_Session__c > ();
    for (Scheduler__C o: Trigger.new) {
        if (o.Type__c == 'Schedule Training') {
            Training_Session__c v = new Training_Session__c();
            v.Training_Module_ID__c = o.Training_Module_ID__c;

            TStoinsert.add(v);
        }

    }
    try {
        insert TStoinsert;
    } catch (system.Dmlexception e) {
        system.debug(e);
    }
}

Answer 1

It would be best to isolate the code that works with the external ID string and keep as much of your logic as possible working with true internal ID values. If that is not possible you should adopt a naming convention to be clear which field is which e.g. Training_Module_ID__c and Training_Module_ExtID__c.

If you can't make those changes, the trigger would need to look something like this where it does the translation from external ID to ID as part of its logic:

trigger createTrainingSession on Scheduler__c(after insert) {

    Set<String> extIds = new Set<String>();
    for (Scheduler__c s: Trigger.new) {
        if (s.Type__c == 'Schedule Training') {
            extIds.add(s.Training_Module_ID__c);
        }
    }

    if (extIds.size() > 0) {

        Map<String, Id> extIdToId = new Map<String, Id>();
        for (Training_Module__c tm : [
                select Id, Training_Module_ID__c
                from Training_Module__c
                where Training_Module_ID__c in :extIds
                ]) {
            extIdToId.put(tm.Training_Module_ID__c, tm.Id);
        }

        List <Training_Session__c> tss = new List <Training_Session__c>();
        for (Scheduler__c s: Trigger.new) {
            if (s.Type__c == 'Schedule Training') {
                tss.add(new Training_Session__c(
                        Training_Module_ID__c = extIdToId.get(s.Training_Module_ID__c)
                        ));
            }
        }
        insert tss;
    }
}