Apex Trigger Scenarios with Solutions

Knowing the syntax is one thing; turning a business requirement into a correct, bulk-safe trigger is another. Here are four scenarios that show up again and again – in real projects and in interviews. Each one is short enough to read in a minute and handles 200 records as easily as one. (To keep each example self-contained, the logic sits in the trigger body; in a real org you'd put it in the handler from the handler lesson.)

Scenario 1: prevent duplicate Leads by email

Requirement: a Lead can't be created or changed to an email that another Lead already has – including duplicates inside the same import.

// LeadTrigger.trigger
trigger LeadTrigger on Lead (before insert, before update) {
    Map<String, Lead> incoming = new Map<String, Lead>();
    for (Lead l : Trigger.new) {
        if (String.isBlank(l.Email)) continue;
        Boolean changed = Trigger.isInsert || l.Email != Trigger.oldMap.get(l.Id).Email;
        if (!changed) continue;
        String key = l.Email.toLowerCase();
        if (incoming.containsKey(key)) {
            l.Email.addError('Duplicate email in this batch: ' + l.Email);   // duplicate inside the batch
        } else {
            incoming.put(key, l);
        }
    }
    if (incoming.isEmpty()) return;

    for (Lead existing : [SELECT Email FROM Lead WHERE Email IN :incoming.keySet() AND Id NOT IN :Trigger.new]) {
        incoming.get(existing.Email.toLowerCase()).Email.addError('A lead with this email already exists.');
    }
}

insert new Lead(LastName = 'Rao', Company = 'Globex', Email = 'neha@globex.example');

List<Lead> batch = new List<Lead>{
    new Lead(LastName = 'Rao', Company = 'Globex', Email = 'NEHA@globex.example'),   // exists already
    new Lead(LastName = 'Shah', Company = 'Initech', Email = 'dev@initech.example'),
    new Lead(LastName = 'Shah 2', Company = 'Initech', Email = 'dev@initech.example') // duplicate in batch
};
List<Database.SaveResult> res = Database.insert(batch, false);
for (Integer i = 0; i < res.size(); i++) {
    System.debug(batch[i].LastName + ': ' + (res[i].isSuccess() ? 'created' : res[i].getErrors()[0].getMessage()));
}

Rao: A lead with this email already exists.
Shah: created
Shah 2: Duplicate email in this batch: dev@initech.example

Matching rules and duplicate rules can do this declaratively too; the Apex version is useful when the logic is more complex (normalised phone numbers, cross-object checks) or must apply to API loads without exceptions.

Scenario 2: Opportunity won → follow-up task and hot account

Requirement: when an Opportunity moves to Closed Won, create an onboarding Task for its owner and mark the Account as Hot – once, only on the change.

// OpportunityTrigger.trigger
trigger OpportunityTrigger on Opportunity (after update) {
    List<Task> tasks = new List<Task>();
    Set<Id> accountIds = new Set<Id>();

    for (Opportunity opp : Trigger.new) {
        Opportunity old = Trigger.oldMap.get(opp.Id);
        if (opp.IsWon && !old.IsWon) {                              // only on the transition
            tasks.add(new Task(Subject = 'Start onboarding: ' + opp.Name, WhatId = opp.Id,
                               OwnerId = opp.OwnerId, ActivityDate = Date.today().addDays(3)));
            if (opp.AccountId != null) accountIds.add(opp.AccountId);
        }
    }
    if (!tasks.isEmpty()) insert tasks;

    List<Account> hot = new List<Account>();
    for (Id accId : accountIds) hot.add(new Account(Id = accId, Rating = 'Hot'));
    if (!hot.isEmpty()) update hot;
}

Account acc = new Account(Name = 'Acme Corp', Rating = 'Cold');
insert acc;
Opportunity opp = new Opportunity(Name = 'Acme renewal', AccountId = acc.Id, StageName = 'Negotiation/Review', CloseDate = Date.today());
insert opp;

opp.StageName = 'Closed Won';
update opp;
opp.Description = 'edited after winning';
update opp;                                                     // no second task: not a transition

System.debug('Tasks: ' + [SELECT COUNT() FROM Task WHERE WhatId = :opp.Id]);
System.debug('Task: ' + [SELECT Subject FROM Task WHERE WhatId = :opp.Id].Subject);
System.debug('Account rating: ' + [SELECT Rating FROM Account WHERE Id = :acc.Id].Rating);

Tasks: 1
Task: Start onboarding: Acme renewal
Account rating: Hot

Scenario 3: block deleting Accounts that have open deals

// AccountTrigger.trigger
trigger AccountTrigger on Account (before delete) {
    Set<Id> withOpenDeals = new Set<Id>();
    for (AggregateResult row : [SELECT AccountId FROM Opportunity
                                WHERE AccountId IN :Trigger.oldMap.keySet() AND IsClosed = false
                                GROUP BY AccountId]) {
        withOpenDeals.add((Id) row.get('AccountId'));
    }
    for (Account acc : Trigger.old) {
        if (withOpenDeals.contains(acc.Id)) {
            acc.addError('This account has open opportunities and cannot be deleted.');
        }
    }
}

Account busy = new Account(Name = 'Busy Ltd');
Account idle = new Account(Name = 'Idle Ltd');
insert new List<Account>{ busy, idle };
insert new Opportunity(Name = 'Open deal', AccountId = busy.Id, StageName = 'Prospecting', CloseDate = Date.today().addDays(30));

List<Database.DeleteResult> res = Database.delete(new List<Account>{ busy, idle }, false);
System.debug('Busy Ltd: ' + (res[0].isSuccess() ? 'deleted' : res[0].getErrors()[0].getMessage()));
System.debug('Idle Ltd: ' + (res[1].isSuccess() ? 'deleted' : res[1].getErrors()[0].getMessage()));

Busy Ltd: This account has open opportunities and cannot be deleted.
Idle Ltd: deleted

In delete triggers there's no Trigger.new – you work with Trigger.old and Trigger.oldMap, and addError on those records blocks the delete.

Scenario 4: copy the Account's address to new Contacts

// ContactTrigger.trigger
trigger ContactTrigger on Contact (before insert) {
    Set<Id> accountIds = new Set<Id>();
    for (Contact c : Trigger.new) {
        if (c.AccountId != null && String.isBlank(c.MailingCity)) accountIds.add(c.AccountId);
    }
    if (accountIds.isEmpty()) return;

    Map<Id, Account> accounts = new Map<Id, Account>(
        [SELECT BillingStreet, BillingCity, BillingPostalCode, BillingCountry FROM Account WHERE Id IN :accountIds]);

    for (Contact c : Trigger.new) {
        Account acc = accounts.get(c.AccountId);
        if (acc == null || String.isNotBlank(c.MailingCity)) continue;
        c.MailingStreet = acc.BillingStreet;
        c.MailingCity = acc.BillingCity;
        c.MailingPostalCode = acc.BillingPostalCode;
        c.MailingCountry = acc.BillingCountry;
    }
}

Account acc = new Account(Name = 'Globex', BillingStreet = '12 MG Road', BillingCity = 'Pune', BillingPostalCode = '411001', BillingCountry = 'India');
insert acc;

List<Contact> contacts = new List<Contact>();
for (Integer i = 1; i <= 200; i++) contacts.add(new Contact(LastName = 'Employee ' + i, AccountId = acc.Id));
contacts.add(new Contact(LastName = 'Remote worker', AccountId = acc.Id, MailingCity = 'Goa'));
insert contacts;

System.debug('Copied: ' + [SELECT COUNT() FROM Contact WHERE MailingCity = 'Pune' AND MailingPostalCode = '411001']);
System.debug('Kept own address: ' + [SELECT MailingCity FROM Contact WHERE LastName = 'Remote worker'].MailingCity);
System.debug('Queries used: ' + Limits.getQueries());

Copied: 200
Kept own address: Goa
Queries used: 3

201 contacts were inserted in one DML, which the platform delivered to the trigger in two chunks (200 + 1). The first chunk needed one query for all 200 accounts' addresses; the second – just the remote worker, who already had a city – returned early without querying. Three queries in total, two of them the script's own checks. The count stays tiny no matter how many contacts arrive.

Practice

Summary