Is it OK to flush the EntityManager from within a domain Service? - orm

Question

I have a domain service called OrderService, with a saveOrder() method:
class OrderService
{
// ...
public function saveOrder(Order $order)
{
$this->orderRepository->add($order);
// $this->entityManager->flush();
$this->notificationService->notifyOrderPlaced($order);
}
}
saveOrder() adds the order to the repository (which internally calls persist() on the EntityManager), then passes the Order to the NotificationService to send appropriate notifications (email, SMS).
The problem is, while NotificationService needs the order ID to include in the notifications, the Order has no ID yet as it's not been persisted to the DB (the ID is auto generated).
The obvious solution seems to pass the EntityManager as a dependency to the OrderService, and flush() right after the repository add() method, as in the example above. But I've always been reluctant to make the domain Services aware of the EntityManager, preferring to let them talk only to repositories, or other services.
What are the drawbacks, if any, of a domain Service having a dependency on the EntityManager?
Is there a better alternative?
Note: I'm using PHP and the Doctrine ORM, but I believe the same principles apply to Java & Hibernate as well.

Answer 1

You may want to consider one of these options (or both)
Make this service an Application layer service instead of a Domain service. It's perfectly OK to call your change tracker in an Application service since it is supposed to know about the application context and progress in the current use case. Typical application services will commit the business transaction/ask the change tracker to save changes when they're done, so why not call it to generate Id's as well ?
If you're concerned about the database being involved in the middle of a use case, maybe you can find an equivalent to NHibernate's Guid.Comb strategy to make your ORM generate an Id without issuing an INSERT to the database right away.
Use a Domain event. Upon creation, an Order could inform the world that it has been newed up. The notification service would handle the event and send appropriate notifications. You'll find an example of that here (it also includes an Application layer service to take care of the business transaction).