Cyclic relationship not reconstituted from DB

[languitar]

I have a model where cycles in the nodes are possible. However, I could not find any way to reconstitute these cycle from the database. Persisting them works as expected.

Here is an example project that demonstrates the problem: https://github.com/languitar/sdn-cycles-repro. A startup event listener executes the reproduction code. This code creates a single node with a relationship pointing to itself. On loading, this cycle is filled with a null value, as can be seen in the output of the reproduction code:

Optional[TestNode{uuid=41ff1326-857c-4d5e-8c05-51626074d14f, cycles=[null]}]

Is there any way to reconstitute such a situation from the database? The documentation is a bit scarce on this topic.

For the reference, this is the node class used:

@Node("TestNode")
public class TestNode {

    @Id
    @GeneratedValue
    private UUID uuid;

    @Relationship("CYCLES_TO")
    private List<TestNode> cycles;

    public TestNode(final UUID uuid, final List<TestNode> cycles) {
        this.uuid = uuid;
        this.cycles = cycles;
    }

    public UUID getUuid() {
        return uuid;
    }

    public void setUuid(UUID uuid) {
        this.uuid = uuid;
    }

    public List<TestNode> getCycles() {
        return cycles;
    }

    public void setCycles(List<TestNode> cycles) {
        this.cycles = cycles;
    }

    @Override
    public String toString() {
        return "TestNode{" +
                "uuid=" + uuid +
                ", cycles=" + cycles +
                '}';
    }
}

[meistermeier]

Sorry for the delayed answer on this. Somehow it slipped through my list.
In this case, you cannot use an all-args constructor (or at least the relationship definition as a parameter). It will end up in a egg-chicken situation, where it wants to construct the object itself but then defers to the creation of the (very same) object first, which defers to the creation... and so on.
Same is true if you have not the same object but something like A(b) and B(a) defined in the constructors. It is not solvable for SDN. You have to manually break the cycle by removing it from the constructor and either providing a wither method (https://docs.spring.io/spring-data/neo4j/docs/current/reference/html/#mapping.fundamentals.property-population) and make it non-final (in your example this is already done).

[languitar]

Thanks for the explanations. Any chance to improve the documentation for this situation? I had experimented with wither methods but didn't think about removing the constructor argument.

[meistermeier]

Will keep this issue open for the documentation.
Currently investigating why a certain exception on load that would tell you about the cyclic dependency, does not get thrown. We introduced the exception here because we don't want people to have an incomplete object tree in the first place when we know that there would be more.

[meistermeier]

FYI
There is already a section about this in the documentation: https://docs.spring.io/spring-data/neo4j/docs/current/reference/html/#mapping.fundamentals.recommendations.note-immutable