Inheritance with Aggregate Roots

If you want to make inheritance work with aggregate roots using a common repository for all subtypes, this can also be achieved very easily.

An example

Consider the following use case:

abstract class User extends \Prooph\EventSourcing\AggregateRoot
{
    protected $name;
    
    protected $email;
    
    public function name(): string
    {
        return $this->name;
    }
    
    public function email(): string
    {
        return $this->email;
    }
    
    protected function apply(AggregateChanged $e): void
    {
        if ($e instanceof UserWasRegisterd) {
            $this->name = $e->name();
            $this->email = $e->email();
        }
    }
}

class Admin extends User
{
    public static function register(string $name, string $email): Admin
    {
        $self = new self();
        $self->recordThat(UserWasRegisterd::withData('admin', $name, $email);
        
        return $self;
    }
}

class Member extends User
{
    public static function register(string $name, string $email): Member
    {
        $self = new self();
        $self->recordThat(UserWasRegisterd::withData('member', $name, $email);
        
        return $self;
    }
}

So in order to make this work, you need 3 small changes in your application.

Step 1: Create a UserAggregateTranslator

final class UserAggregateTranslator implements \Prooph\EventSourcing\Aggregate\AggregateTranslator
{
    /**
     * @var AggregateRootDecorator
     */
    protected $aggregateRootDecorator;

    /**
     * @param object $eventSourcedAggregateRoot
     *
     * @return int
     */
    public function extractAggregateVersion($eventSourcedAggregateRoot): int
    {
        return $this->getAggregateRootDecorator()->extractAggregateVersion($eventSourcedAggregateRoot);
    }

    /**
     * @param object $anEventSourcedAggregateRoot
     *
     * @return string
     */
    public function extractAggregateId($anEventSourcedAggregateRoot): string
    {
        return $this->getAggregateRootDecorator()->extractAggregateId($anEventSourcedAggregateRoot);
    }

    /**
     * @param AggregateType $aggregateType
     * @param Iterator $historyEvents
     *
     * @return object reconstructed AggregateRoot
     */
    public function reconstituteAggregateFromHistory(AggregateType $aggregateType, Iterator $historyEvents)
    {
        $aggregateRootDecorator = $this->getAggregateRootDecorator();

        $firstEvent = $historyEvents->current();
        $type = $firstEvent->type();

        if ($type === 'admin') {
            return $aggregateRootDecorator->fromHistory(Admin::class, $historyEvents);
        } elseif ($type === 'member') {
            return $aggregateRootDecorator->fromHistory(Member::class, $historyEvents);
        }
    }

    /**
     * @param object $anEventSourcedAggregateRoot
     *
     * @return Message[]
     */
    public function extractPendingStreamEvents($anEventSourcedAggregateRoot): array
    {
        return $this->getAggregateRootDecorator()->extractRecordedEvents($anEventSourcedAggregateRoot);
    }

    /**
     * @param object $anEventSourcedAggregateRoot
     * @param Iterator $events
     *
     * @return void
     */
    public function replayStreamEvents($anEventSourcedAggregateRoot, Iterator $events): void
    {
        $this->getAggregateRootDecorator()->replayStreamEvents($anEventSourcedAggregateRoot, $events);
    }

    public function getAggregateRootDecorator(): AggregateRootDecorator
    {
        if (null === $this->aggregateRootDecorator) {
            $this->aggregateRootDecorator = AggregateRootDecorator::newInstance();
        }

        return $this->aggregateRootDecorator;
    }

    public function setAggregateRootDecorator(AggregateRootDecorator $anAggregateRootDecorator): void
    {
        $this->aggregateRootDecorator = $anAggregateRootDecorator;
    }
}

Step 2: Change the assertion method in the EventStoreUserCollection

final class EventStoreUserCollection extends 
    \Prooph\EventStore\Aggregate\AggregateRepository
{
    public function save(User $user): void
    {
        $this->saveAggregateRoot($user);
    }
    public function get(UserId $userId): ?User
    {
        return $this->getAggregateRoot($userId->toString());
    }
    protected function assertAggregateType($eventSourcedAggregateRoot)
    {
        \Assert\Assertion::isInstanceOf($eventSourcedAggregateRoot, User::class);
    }
}

Step 3: Make use of your custom AggregateTranslator

final class EventStoreUserCollectionFactory
{
    public function __invoke(ContainerInterface $container): EventStoreUserCollection
    {
        return new EventStoreUserCollection(
            $container->get(EventStore::class),
            AggregateType::fromAggregateRootClass(User::class),
            new UserAggregateTranslator()
        );
    }
}

If you use the provided container factory (\Prooph\EventStore\Container\Aggregate\AbstractAggregateRepositoryFactory) then you can also just change the aggregate_translator key in your config to point to the new UserAggregateTranslator and register the UserAggregateTranslator in your container.

see also: http://www.sasaprolic.com/2016/02/inheritance-with-aggregate-roots-in.html

Alternative to AggregateRoot inheritance

Abstract Prooph\EventSourcing\AggregateRoot class provides a solid basis for your aggregate roots, however, it is not mandatory. Two traits, Prooph\EventSourcing\Aggregate\EventProducerTrait and Prooph\EventSourcing\Aggregate\EventSourcedTrait, together provide exactly the same functionality.

• EventProducerTrait is responsible for event producing side of Event Sourcing and might be used independently of EventSourcedTrait when you are not ready to start with full event sourcing but still want to get the benefit of design validation and audit trail provided by Event Sourcing. Forcing all changes to be applied internally via event sourcing will ensure events data consistency with the state and will make it easier to switch to full event sourcing later on.

• EventSourcedTrait is responsible for restoring state from event stream, it should be used together with EventProducerTrait as normally you will not be applying events not produced by that aggregate root.

Default aggregate translator uses AggregateRootDecorator to access protected methods of Prooph\EventSourcing\AggregateRoot descendants, you will need to switch to Prooph\EventSourcing\EventStoreIntegration\ClosureAggregateTranslator for aggregate roots using traits.