Join queries aren't using same "root" instance

[Goldmensch]

@MappedEntity
public record Cart(
    @Id @GeneratedValue @Nullable
    Long id,
    String brand,
    @Relation(value = Kind.ONE_TO_MANY, mappedBy = "cart", cascade = Cascade.ALL)
    List<CartPart> parts) {}

@MappedEntity()
public record CartPart(
    @Id @GeneratedValue @Nullable
    Long id,
    String type,
    @Relation(Kind.MANY_TO_ONE) @Nullable
    Cart cart
) {}

@JdbcRepository
public interface TestRepo extends CrudRepository<Cart, Long> {
  @Override
  @Join(value = "parts")
  Optional<Cart> findById(@Id @Nullable Long id);
}

So when I fetch a Cart via findById then each CartPart has a different new instance of Cart and this instance hasn't any parts.
Is there any way to have Micronaut reuse the same instance in each CartPart?

test:

    var cart = new Cart(null, "VW", List.of(
        new CartPart(null, "door", null),
        new CartPart(null, "motor", null)));

    var saved = repo.save(cart);
    var foundCart = repo.findById(saved.id()).orElseThrow();

    System.out.println(foundCart);
    System.out.println(foundCart.parts().get(0).cart());
    System.out.println(System.identityHashCode(foundCart));
    System.out.println(System.identityHashCode(foundCart.parts().get(0)));
    System.out.println(System.identityHashCode(foundCart.parts().get(1)));

output:

Cart[id=1, brand=VW, parts=[CartPart[id=1, type=door, cart=Cart[id=1, brand=VW, parts=[]]], CartPart[id=2, type=motor, cart=Cart[id=1, brand=VW, parts=[]]]]]
Cart[id=1, brand=VW, parts=[]]
1899145692
1464031233
765880900
Cart[id=1, brand=VW, parts=[CartPart[id=1, type=door, cart=Cart[id=1, brand=VW, parts=[]]], CartPart[id=2, type=motor, cart=Cart[id=1, brand=VW, parts=[]]]]]

[dstepanov]

You should probably post the output of:

System.out.println(System.identityHashCode(foundCart.parts().get(0).cart()));
System.out.println(System.identityHashCode(foundCart.parts().get(1).cart()));

When using immutable records you cannot simply modify the parent and have the children reference the updated parent.

[Goldmensch]

I'm sorry for being late, the output of this is:

1047000562
265629779
265629779

    System.out.println(System.identityHashCode(foundCart));
    System.out.println(System.identityHashCode(foundCart.parts().get(0).cart()));
    System.out.println(System.identityHashCode(foundCart.parts().get(1).cart()));

So it seems that Micronaut only fetches/creates one new instance without any parts. That's a good message. But is there any way to let Micronaut use the parent instance already given, so that Micronaut uses the "foundCart" instance in the childs as well, and thus foundCart is the same as foundCart.parts().get(n).cart()?

[dstepanov]

But is there any way to let Micronaut use the parent instance already given

It isn't possible when dealing with immutable structures because every modification requires a new instance. To set the parent into the child you need to modify the child and then you need to update the child instance in the parent etc

[Goldmensch]

But is there any way to let Micronaut use the parent instance already given

It isn't possible when dealing with immutable structures because every modification requires a new instance. To set the parent into the child you need to modify the child and then you need to update the child instance in the parent etc

well, seems logical. Thank you anyway