Service Layer

public with sharing class TransactionValidationService {
    public static void validate(List<Transaction__c> transactions, Map<Id, Bank_Account__c> accountsById) {
        for (Transaction__c t : transactions) {
            Bank_Account__c acc = accountsById.get(t.Bank_Account__c);
            if (acc.Status__c != 'Active') {
                t.addError('This account is not active and cannot accept transactions.');
            } else if (t.Type__c == 'Withdrawal' && t.Amount__c > acc.Balance__c) {
                t.addError('Insufficient funds for this withdrawal.');
            }
        }
    }
}

The exact same business rule from Module 42's TransactionValidation trigger, moved into a class with no dependency on Trigger.new or Trigger.old — it takes plain lists and maps as parameters instead, which is precisely what makes it testable in isolation.

trigger TransactionValidation on Transaction__c (before insert) {
    Set<Id> accountIds = new Set<Id>();
    for (Transaction__c t : Trigger.new) accountIds.add(t.Bank_Account__c);

    Map<Id, Bank_Account__c> accounts = new Map<Id, Bank_Account__c>(
        [SELECT Id, Balance__c, Status__c FROM Bank_Account__c WHERE Id IN :accountIds]
    );

    TransactionValidationService.validate(Trigger.new, accounts);
}

The trigger file now only handles trigger mechanics — collecting IDs and calling the service — while the actual business decision lives in TransactionValidationService, satisfying Lesson 2's separation of concerns directly.

@isTest
static void rejectsInactiveAccountTransaction() {
    Bank_Account__c acc = new Bank_Account__c(Id = TestHelper.fakeId(Bank_Account__c.SObjectType), Status__c = 'Frozen', Balance__c = 100);
    Transaction__c t = new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Deposit', Amount__c = 10);

    TransactionValidationService.validate(new List<Transaction__c>{t}, new Map<Id, Bank_Account__c>{acc.Id => acc});

    Assert.isTrue(t.hasErrors());
}

Because TransactionValidationService.validate accepts plain data rather than needing an actual DML insert to trigger it, this test never needs to insert a real Bank_Account__c record — a meaningfully faster and simpler test than Module 42's original approach, which had to insert real records to exercise the trigger.