Lesson: 28 | Avoid deletion of the account with a contact

Apex Trigger

trigger AccountTrigger on Account (before delete) {
    List<Contact> relatedContacts =[SELECT AccountId FROM Contact WHERE AccountId IN: Trigger.oldMap.keySet()];
    for(Contact con : relatedContacts){
        Trigger.oldMap.get(con.AccountId).addError('You can not delete an Account with a Contact');
    }
}