Add association support for DTO projections

[pelletier197]

Given I have an entity that has a single relationship to another entity. If I want to have a projection of that entity with a nullable related entity, the request fails.

interface TestPersonRepository : Neo4jRepository<PersonEntity, String> {
    @Query("""
        MATCH (person:Person) 
        OPTIONAL MATCH (person)-[:WORKS_IN]->(department:Department)
        return person, department
    """)
    fun findPeopleWithDepartments() : List<PersonProjection>
}

data class PersonProjection(
    val person: PersonEntity,
    val department: DepartmentEntity?
)

@Service
class TestService(
    private val repository: TestPersonRepository
)
{
    init {
            val results = repository.findPeopleWithDepartments() // Throws exception
            println(results) 
    }
}

I actually saw a related issue that is marked as resolved, but I think the null use-case was forgotten in 6.1.2: #2269

The stacktrace I get is

Caused by: org.neo4j.driver.exceptions.value.NotMultiValued: NULL is not iterable
	at org.neo4j.driver.internal.value.ValueAdapter.values(ValueAdapter.java:401)
	at org.neo4j.driver.internal.value.ValueAdapter.values(ValueAdapter.java:395)
	at org.springframework.data.neo4j.core.mapping.DefaultNeo4jEntityConverter.determineQueryRoot(DefaultNeo4jEntityConverter.java:127)
	at org.springframework.data.neo4j.core.mapping.DefaultNeo4jEntityConverter.read(DefaultNeo4jEntityConverter.java:102)
	at org.springframework.data.neo4j.core.mapping.DefaultNeo4jEntityConverter.read(DefaultNeo4jEntityConverter.java:67)
	at org.springframework.data.neo4j.core.mapping.DtoInstantiatingConverter.lambda$getPropertyValueFor$4(DtoInstantiatingConverter.java:160)
	at org.springframework.data.neo4j.core.mapping.DtoInstantiatingConverter.getPropertyValueFor(DtoInstantiatingConverter.java:173)
	at org.springframework.data.neo4j.core.mapping.DtoInstantiatingConverter$1.getParameterValue(DtoInstantiatingConverter.java:102)
	at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.extractInvocationArguments(ClassGeneratingEntityInstantiator.java:269)

The source of this is in DefaultNeo4jEntityConverter.

Iterable<Value> recordValues = mapAccessor instanceof Value && ((Value) mapAccessor).hasType(nodeType) ?
		Collections.singletonList((Value) mapAccessor) : mapAccessor.values(); // <-- this last part throws when the single entity result is null, since the mapAccessor is a `NullValue`.

[meistermeier]

Thanks for reporting this. It was not really forgotten but more something that we just had not on our list at all (now it is).

[meistermeier]

TBH I cannot reproduce the particular issue, you are facing...but
Your DTO projection is based on the wrong assumptions: The PersonProjection should already define the PersonEntity fields instead of encapsulating it. An example can be seen here https://github.com/spring-projects/spring-data-neo4j/compare/6.1.x...issue/gh-2305#diff-e7c31326d3475b6bb745de782319b4c3c3261bff1651348184a52eead5aa9cd8R43
Also I added a test case for your scenario, or at least how I understand it: https://github.com/spring-projects/spring-data-neo4j/compare/6.1.x...issue/gh-2305#diff-870bdc2d6110e1d562badf679618d645bcdd33d31c21bf16cc505f528f99b3c6R1292
With this you would also have to return the relationship from the query to not just join two arbitrary nodes in the projection object but only ones with the right relationship type.

If you could clarify how my example differs from yours, this would be really helpful to give you more input on your problem.

Your question also helped us to uncover another missing behaviour around the DTO projections, thanks.

[pelletier197]

The links you provided don't seem to work for me. However, I looked at the code on your branch and I think that the difference with my example is that the PersonProjection is not annotated with @Node, in comparison to the OneToOneSource.

Also, I really don't see why passing the relation would be necessary here? My projection object does not specify any relation in it. I want to project a person with a possibly null department. What if there was more than one relation to get from person to department?

========

The PersonProjection should already define the PersonEntity fields instead of encapsulating it.

I don't know if it's an official feature, but encapsulation does work (except with my use-case of a null one to one projection). I find it much more convenient to use encapsulation than copying 15 fields of Person, don't you think?

[meistermeier]

(Updated the links above, somehow the GitHub magic creates something "interesting" out of this link format)
A projection is based on a modeled entity. You cannot project some independent values in the context of a domain / entity specific operation. And as a consequent only existing fields from the original entity are valid to get hydrated.
(Note: there are some special cases where you can use Spring Expression Language but this is out of this scope here)
If you want to have also the related node in there, Spring Data Neo4j needs to know "how" (relationship type) they are connected. This is the reason why the relationship needs to get returned by the query.
The projection itself is also not annotated with @Node in the example I am referring to but shows a valid DTO projection.

If you really, really want to have this objects, you would have to use the Neo4jClient and map those fields independently.
This would be similar to #2288 (comment)