Logical cyclic mapping dependency

[damienthewave]

Hi, yesterday I ran into a problem while creating an account in the application I'm currently developing. The console log says:

org.springframework.data.mapping.MappingException: The node with id 5 has a logical cyclic mapping dependency. Its creation caused the creation of another node that has a reference to this.

Classes related:

@Node("Tree")
class Tree(
    @Relationship(type = "OWNS_TREE", direction = Relationship.Direction.INCOMING)
    val owner: Person
) {
    @Id
    @GeneratedValue
    var id: Long = -1

    ...
}

@Node("Person")
class Person(
    var firstName: String,   
    ) {
    @Id
    @GeneratedValue
    var personId: Long = -1

    @Relationship(type = "IS_CHILD_OF", direction = Relationship.Direction.OUTGOING)
    var parents: MutableSet<Person> = mutableSetOf()

    @Relationship(type = "IS_CHILD_OF", direction = Relationship.Direction.INCOMING)
    var children: MutableSet<Person> = mutableSetOf()

    @Relationship(type = "IS_SIBLING_OF")
    var siblings: MutableSet<Person> = mutableSetOf()

    @Relationship(type = "IS_PARTNER_OF")
    var partners: MutableSet<Person> = mutableSetOf()

    @Relationship(type = "OWNS_TREE")
    var ownedTree: Tree? = null

I know that this is because in Person there is a OWNS_TREE relationship, that is also in Tree, which would make it a cyclic dependency, but what does it mean exactly?
The second question is how should I process to having the relationship between mentioned classes that way and working on them without getting that kind of an exception?

[gerrit_meier · Accepted solution]

The problem is rooted in the dependency chain (cycle) Tree -> Person -> Tree, as you have already discovered.
Spring Data Neo4j has a dead-lock detection during mapping to avoid stack overflow / infinite recursion if it comes to cyclic mapping dependencies. This is why you see the exception.

What happens in detail?

1. SDN starts mapping the Tree that depends on a Person instance (constructor) and puts it in a set of "objects in construction".
2. The Person instance gets created AND the properties of it populated. This has to be done because a property can -besides public visibility and setter- also be "set" by using a wither method (Spring Data Neo4j)
3. The ownedTree property needs the Tree-in-construction -> Exception

To avoid this, you can either remove the constructor dependency or the bi-directional relationship definition.