Apex Triggers Tutorial: Events and Context Variables

A trigger is Apex that runs automatically when records are inserted, updated, deleted or undeleted – whether the change comes from a user, an import, an API call or a Flow. Triggers are where much of an org's business logic lives, which is why they're the most important – and most interviewed-on – part of Apex.

Syntax and events

trigger TriggerName on ObjectName (before insert, before update, before delete,
                                   after insert, after update, after delete, after undelete) {
    // code
}

A first trigger

// AccountTrigger.trigger
trigger AccountTrigger on Account (before insert, before update, after insert) {
    if (Trigger.isBefore) {
        for (Account acc : Trigger.new) {
            acc.Name = acc.Name.trim();                         // change the record itself – no DML
            if (acc.Rating == null) acc.Rating = 'Warm';        // default a field
        }
        System.debug('before ' + (Trigger.isInsert ? 'insert' : 'update') + ': ' + Trigger.size + ' record(s)');
    }

    if (Trigger.isAfter && Trigger.isInsert) {
        List<Task> followUps = new List<Task>();
        for (Account acc : Trigger.new) {                       // Ids exist now
            followUps.add(new Task(Subject = 'Welcome call: ' + acc.Name, WhatId = acc.Id, Priority = 'Normal'));
        }
        insert followUps;                                       // DML on OTHER records: after trigger
        System.debug('after insert: created ' + followUps.size() + ' task(s)');
    }
}

List<Account> accs = new List<Account>{
    new Account(Name = '  Acme Corp  '),
    new Account(Name = 'Globex', Rating = 'Hot')
};
insert accs;

for (Account a : [SELECT Name, Rating, (SELECT Subject FROM Tasks) FROM Account ORDER BY Name]) {
    System.debug('"' + a.Name + '" rating ' + a.Rating + ', tasks: ' + a.Tasks[0].Subject);
}

accs[0].Rating = null;
update accs[0];
System.debug('Rating after update: ' + [SELECT Rating FROM Account WHERE Id = :accs[0].Id].Rating);

before insert: 2 record(s)
after insert: created 2 task(s)
"Acme Corp" rating Warm, tasks: Welcome call: Acme Corp
"Globex" rating Hot, tasks: Welcome call: Globex
before update: 1 record(s)
Rating after update: Warm

The before trigger trimmed the name and defaulted the rating without any DML – changes to Trigger.new in a before trigger are saved automatically. The after trigger, running once the records had Ids, created related Tasks. One trigger fired once for both accounts: triggers always receive records in bulk.

Context variables

Trigger.newMap isn't available in before insert – the records have no Ids yet. And in after triggers, Trigger.new is read-only: to change those records you'd need DML, which is why field changes belong in before triggers.

Detecting changes and blocking saves

Compare new and old values to react only when a field actually changes, and use addError to stop a save with a message the user sees:

// OpportunityTrigger.trigger
trigger OpportunityTrigger on Opportunity (before update) {
    for (Opportunity opp : Trigger.new) {
        Opportunity old = Trigger.oldMap.get(opp.Id);

        if (old.StageName == 'Closed Won' && opp.StageName != 'Closed Won') {
            opp.StageName.addError('A won deal cannot be reopened. Create a new opportunity instead.');
            continue;
        }
        if (opp.Amount != old.Amount && opp.Amount < old.Amount * 0.5) {
            opp.addError('Amount cannot drop by more than half in one edit.');
        }
    }
}

Opportunity won = new Opportunity(Name = 'Acme renewal', StageName = 'Closed Won', CloseDate = Date.today(), Amount = 100000);
Opportunity open = new Opportunity(Name = 'Globex pilot', StageName = 'Prospecting', CloseDate = Date.today(), Amount = 80000);
insert new List<Opportunity>{ won, open };

won.StageName = 'Negotiation/Review';
open.Amount = 20000;
Opportunity fine = new Opportunity(Id = open.Id, Description = 'notes only');

List<Database.SaveResult> results = Database.update(new List<Opportunity>{ won, open }, false);
for (Database.SaveResult r : results) {
    System.debug(r.isSuccess() ? 'saved' : 'blocked: ' + r.getErrors()[0].getMessage() + ' ' + r.getErrors()[0].getFields());
}
update fine;
System.debug('A change that breaks no rule saves normally');

blocked: A won deal cannot be reopened. Create a new opportunity instead. (StageName)
blocked: Amount cannot drop by more than half in one edit. ()
A change that breaks no rule saves normally

• field.addError() shows the message next to the field in the UI; record.addError() at the top of the page.
• addError doesn't throw: the trigger keeps running, and the record simply fails to save. With partial-success DML the other records still save.
• Comparing with Trigger.oldMap makes the rule fire only on real changes, not on every edit.

Practice

Summary