BeforeConvertCallback<> not called when persisting nested entity

[polyoxidonium]

Due to different database engines I chose to generate UUID within application and not database so I used this callback method.

So consider entities:

public class Order implements Persistable {

    @Id
    private UUID orderId;
    private OrderType orderType;
    @MappedCollection(idColumn = "order_id")
    private Set<Product> products;
...
}

public class Product implements Persistable {
    @Id
    private UUID productId;
    private String productType;
    private String productName;
    private UUID orderId;
...
}

and callback:

@Component
class EntityCallback implements BeforeConvertCallback<Persistable> {

    @Override
    public Persistable onBeforeConvert(Persistable aggregate) {
        if (aggregate.getEntityId() == null) {
            aggregate.setEntityId(UUID.randomUUID());
        }
        return aggregate;
    }
}

when I execute orderRepository.save(order)
I get following exception
SQLException with SQL state '23502', error code '23502', message [NULL not allowed for column "PRODUCT_ID";

meaning that productId was not generated during callback function (checked via debug)
Is this intended to work like this?

[schauder]

Yes, this is intended. Events and callbacks are only triggered for aggregates.

Unfortunately this is not properly documented.
We'll fix that.