How to express a link to self

[MehrCurry]

I want to have an Aggregate with a parent/child relation to self.

@RequiredArgsConstructor
@AllArgsConstructor
@Getter
public class Location implements AggregateRoot<Location, Location.LocationId> {
    private final LocationId id;

    private final LocationName name;

    @Nullable
    private Location parent;

    @Value(staticConstructor = "of")
    public static class LocationId implements Identifier {
        UUID id;

        public static final LocationId random() {
            return Location.LocationId.of(UUID.randomUUID());
        }
    }
}

Saving the (empty) root object (with JPA) works. But when i save a child of that root, JPA tries to create the root again which leads to an error.

Hibernate: 
    insert 
    into
        location
        (name, parent_id, id) 
    values
        (?, ?, ?)
2022-05-01 07:50:07.656 TRACE 53749 --- [  restartedMain] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [VARCHAR] - [Liegenschaft]
2022-05-01 07:50:07.656 TRACE 53749 --- [  restartedMain] o.h.type.descriptor.sql.BasicBinder      : binding parameter [2] as [BINARY] - [null]
2022-05-01 07:50:07.657 TRACE 53749 --- [  restartedMain] o.h.type.descriptor.sql.BasicBinder      : binding parameter [3] as [BINARY] - [6a3c3f9e-33e3-4fd7-b9e1-cc7be2cd536c]
2022-05-01 07:50:07.661  WARN 53749 --- [  restartedMain] o.h.engine.jdbc.spi.SqlExceptionHelper   : SQL Error: 23505, SQLState: 23505
2022-05-01 07:50:07.661 ERROR 53749 --- [  restartedMain] o.h.engine.jdbc.spi.SqlExceptionHelper   : Unique index or primary key violation: "LIEGENSCHAFT.PRIMARY_KEY_9 ON LIEGENSCHAFT.LOCATION(ID) VALUES 1"; SQL statement:
insert into location (name, parent_id, id) values (?, ?, ?) [23505-200]

So how do i express recursive links?

[odrotbohm]

Sorry for not seeing this earlier: the reason for the effect you see is that Location – by definition – is an Entity. Entities are "owned" by the aggregates they're contained in, i.e. no two aggregates must refer to the same entity as state changes issued by one aggregate might affect the consistency of the other. Thus, a @OneToOne mapping is the only reasonable way to map singular properties that are entities.

That said, the entity referred to is also an AggregateRoot. Following the pure theory, references to an aggregate either should be identifier references or modeled as an Association. That takes the relationship out of the loop for Hibernate's automatic relationship management, as it is reduced to a "simple property". Switching to that model should resolve the issue but of course changes the picture as you cannot query into properties of the related aggregate anymore.

If you really want to get this going as is, you should be able to annotate the parent property with @ManyToOne which disables the generation of the default mapping described above.