Clear exception message on recursive mapping of immutable objects.

[Andy2003]

I've created a Test case showing a StackOverflowError at https://github.com/Andy2003/spring-data-neo4j/tree/complex-test (Andy2003@d54e4be)

The error occurs when in a parent-child mapping the relation is modeled for both directions, like:

@org.springframework.data.neo4j.core.schema.Node
public abstract class BaseNodeEntity {
	@Relationship(type = "CHILD_OF", direction = OUTGOING)
	BaseNodeEntity childOf;
}

and

@org.springframework.data.neo4j.core.schema.Node
public class Node extends BaseNodeEntity {
	@Relationship(type = "CHILD_OF", direction = INCOMING)
	protected Set<BaseNodeEntity> childNodes;
}

and data like:

CREATE (root:Node:BaseNodeEntity{nodeId: 'root'})
CREATE (company:Node:BaseNodeEntity{nodeId: 'comp'})
CREATE (company)-[:CHILD_OF]->(root)

[meistermeier]

You are using the Lombok library with @AllArgsConstructor(onConstructor = @__(@PersistenceConstructor)) for the BaseNodeEntity. Combined with the manual all args @PersistenceConstructor in the Node you create a hen/egg scenario.
The mapping process wants to create the "root" Node but for this it needs the "comp" Node which depends again on the not-yet-complete-constructed "root" Node.
We will investigate if we could detect this incoming data-wise before running into the SO and log a warning or similar.

[Andy2003]

Is it possible, that you inject the not jet fully populated object into the "child"-constructor, so you won't run into the cycle?
I used immutable Objects as the docs suggested it as best practice

[meistermeier]

No, we tried this more than once, but it is not possible to do this in any way.
For true immutability the root object for example has to be the same as the root object and the one you can reach by the parent/child object. Every approach failed because it is just not possible to assemble this.

[Andy2003]

I found this on StackOverflow https://stackoverflow.com/a/4834584/1666829
I guess I have to care about this scenario in client code and use an approach similar to the one described in the post.