Testing aggregates

In the test suite of your application you probably also want to write some tests to check if an aggregate works correctly. The package contains some handy methods to help you.

Imagine you have an AccountAggregateRoot that handles adding and subtract an amount for a bank account. The account has a limit of -$5000.

use Spatie\EventSourcing\AggregateRoots\AggregateRoot;

class AccountAggregateRoot extends AggregateRoot
{
    /** @var int */
    private $balance = 0;

    /** @var int */
    private $accountLimit = -5000;

    public function createAccount(string $name, string $userId)
    {
        $this->recordThat(new AccountCreated($name, $userId));

        return $this;
    }

    public function addMoney(int $amount)
    {
        $this->recordThat(new MoneyAdded($amount));

        return $this;
    }

    protected function applyMoneyAdded(MoneyAdded $event)
    {
        $this->balance += $event->amount;
    }

    public function subtractMoney(int $amount)
    {
        $this->hasSufficientFundsToSubtractAmount($amount)
            ? $this->recordThat(new MoneySubtracted($amount))
            : $this->recordThat(new AccountLimitHit($amount));
    }

    protected function applyMoneySubtracted(MoneySubtracted $event)
    {
        $this->balance -= $event->amount;
    }

    private function hasSufficientFundsToSubtractAmount(int $amount): bool
    {
        return $this->balance - $amount >= $this->accountLimit;
    }
}

Let's now test that rule that an account cannot go beyond its limit.

// in a PHPUnit test

/** @test */
public function it_can_subtract_money()
{
    AccountAggregateRoot::fake()
        ->given([new AccountCreated('Luke'), new MoneySubtracted(4999)])
        ->when(function (AccountAggregate $accountAggregate) {
            $accountAggregate->subtractMoney(1);
        })
        ->assertRecorded(new MoneySubtracted(1))
        ->assertNotRecorded(AccountLimitHit::class);
}

/** @test */
public function it_will_not_make_subtractions_that_would_go_below_the_account_limit()
{
    AccountAggregateRoot::fake()
        ->given([new AccountCreated('Luke'), new MoneySubtracted(4999)])
        ->when(function (AccountAggregate $accountAggregate) {
            $accountAggregate->subtractMoney(2);
        })
        ->assertRecorded(new AccountLimitHit(2))
        ->assertNotRecorded(MoneySubtracted::class);
}

The given and assertRecorded methods can accept a single event instances or an array with event instances. assertNotRecorded can also accept an array of class names.

When you'd like to assert that only a specific event is recorded, you can use the assertEventRecorded method.

If you don't expect any events to be recorded you can use assertNothingRecorded.

Finally, you can also use then, it accepts a closure which is passed any input you've returned from the when function:

/** @test */
public function it_will_not_make_subtractions_that_would_go_below_the_account_limit()
{
    AccountAggregateRoot::fake()
        ->given([new AccountCreated('Luke'), new MoneySubtracted(4999)])
        ->when(function (AccountAggregate $accountAggregate) {
            $accountAggregate->subtractMoney(2);

            return $accountAggregate->uuid();
        })
        ->then(function($aggregateRootUuid) {
            // …
        });
}

You can choose to do manually perform assertions in the then closure. Alternatively, you can return true or false from. Returning false will fail the test.

##Disabling dispatching events

When calling the given method the aggregate will fire of events for your projector and reactor to react to. If you don't want events being dispatched. Simply use the Event facade's fake method before your test executes.

\Illuminate\Support\Facades\Event::fake();

If you would prefer to disable Projectors reacting to events whilst retaining Event functionality simply use the Projectionist facade's withoutEventHandlers method before your test executes.

use Spatie\EventSourcing\Facades\Projectionist;
Projectionist::withoutEventHandlers();

##Want to know more?

Aggregate roots are a crucial part in large applications. Our course, Event Sourcing in Laravel covers them in depth:

• 9. Aggregate Roots
• 10. State Management in Aggregate Roots
• 11. Multi-Entity Aggregate Roots
• 12. State Machines with Aggregate Entities