You sometimes need to add restrictions on certain database operations, such as preventing records from being saved when certain conditions are met. To prevent saving records in a trigger, call the addError() method on the sObject in question. The addError() method throws a fatal error inside a trigger. The error message is displayed in the user interface and is logged.

The following trigger prevents the deletion of an account if it has related opportunities. By default, deleting an account causes a cascade delete of all its related records. This trigger prevents the cascade delete of opportunities.

trigger AccountDeletion on Account (before delete) {
   
    // Prevent the deletion of accounts if they have related opportunities.
    for (Account a : [SELECT Id FROM Account
                     WHERE Id IN (SELECT AccountId FROM Opportunity) AND
                     Id IN :Trigger.old]) {
        Trigger.oldMap.get(a.Id).addError(
            'Cannot delete account with related opportunities.');
    }
    
}

Calling addError() in a trigger causes the entire set of operations to roll back, except when bulk DML is called with partial success.

• If a DML statement in Apex spawned the trigger, any error rolls back the entire operation. However, the runtime engine still processes every record in the operation to compile a comprehensive list of errors.
• If a bulk DML call in the Lightning Platform API spawned the trigger, the runtime engine sets the bad records aside. The runtime engine then attempts a partial save of the records that did not generate errors.