Aggregate root with aggregates

Sometimes you might want to split up your aggregate into multiple partial aggregates, while maintaining the transactionality of one parent aggregate.

To add this functionality, you’d use the AggregateRootWithAggregates trait in your parent aggregate root. Your child aggregate root(s) need to implement the EventSourcedAggregate interface. You can use the AggregateAppliesKnownEvents trait in order to apply the events that are relevant for the partial.

In order to apply events to your child aggregates, you need to register the child using the $this->registerAggregate($this->yourChildAggregate) method in your parent aggregate.

When you want to remove the aggregate, you can use the $this->unregisterAggregate($this->yourChildAggregate) method. This will stop the aggregate from receiving new events to apply.

As example, lets imagine we have a ShoppingCart aggregate that manage the items inside a cart. We could put the logic that deals with adding and removing items to a sub aggregate: CartItems.

It’s important to note, that we’d still only call methods on the ShoppingCart Aggregate. A naive, simple example of a shopping cart with a CartItems sub aggregate could look like this:

class ShoppingCart implements AggregateRoot
{
    use AggregateRootWithAggregates;

    private CartItems $cartItems;

    public static function initiate(ShoppingCartId $id): static
    {
        $shoppingCart = new static($id);
        $shoppingCart->recordThat(new ShoppingCartInitiated());

        return $shoppingCart;
    }

    public function addProduct(ProductId $productId, int $currentPriceInCents): void
    {
        $this->cartItems->add($productId, $currentPriceInCents);
    }

    public function checkout(): void
    {
        if ($this->cartItems->isEmpty()) {
            throw SorryCantCheckout::becauseThereAreNoProductsInCart();
        }
        $this->recordThat(new CartCheckedOut());
    }

    protected function applyShoppingCartInitiated(ShoppingCartInitiated $event): void
    {
        $this->bootCartItemsAggregate();
    }

    private function bootCartItemsAggregate(): void
    {
        $this->cartItems = new CartItems($this->eventRecorder());
        $this->registerAggregate($this->cartItems);
    }
}

class CartItems implements EventSourcedAggregate
{
    use AggregateAppliesKnownEvents;

    private array $items = [];

    public function __construct(private EventRecorder $eventRecorder)
    {
    }

    public function add(ProductId $productId, int $currentPriceInCents): void
    {
        if (array_key_exists($productId->toString(), $this->items)) {
            if ($this->items[$productId->toString()]['price'] !== $currentPriceInCents) {
                throw SorryCantAddProduct::becauseThePriceHasChanged();
            }
        }
        $this->eventRecorder->recordThat(new ProductAddedToCart($productId, $currentPriceInCents));
    }

    public function isEmpty(): bool
    {
        return count($this->items) === 0;
    }

    protected function applyProductAddedToCart(ProductAddedToCart $event): void
    {
        $productIdAsString = $event->productId->toString();
        if ( ! array_key_exists($productIdAsString, $this->items)) {
            $this->items[$productIdAsString] = ['qty' => 0, 'price' => $event->price];
        }
        ++$this->items[$productIdAsString]['qty'];
    }
}