多级投影上的 Spring Data Neo4J 层次映射

Question

I have a simple hierarchy in neo4j directly derived from the business model.

@Node
public class Team {
    @Id private String teamId;
    private String name;
}
@Node
public class Driver {
    @Id private String driverId;
    private String name;
    @Relationship(direction = Relationship.Direction.OUTGOING)
    private Team team;
}
@Node
public class Car {
    @Id private String carId;
    private String name;
    @Relationship(direction = Relationship.Direction.OUTGOING)
    private Driver driver;
}

which results in the corresponding graph (Team)<--(Driver)<--(Car) usually all requests start at Car .
A new use case needs to create a tree structure starting at Team nodes. The Cypher query aggregates the data on neo and returns it to SDN.

public List<Projection> loadHierarchy() {
    return neo4jClient.query("""
                MATCH(t:Team)<--(d:Driver)<--(c:Car)
                WITH t, d, collect(distinct c{.carId, .name}) AS carsEachDriver
                WITH t, collect({driver: d{.driverId, .name}, cars: carsEachDriver }) AS driverEachTeam
                WITH collect({team: t{.teamId, .name}, drivers: driverEachTeam }) as teams
                RETURN teams
             """)
            .fetchAs(Projection.class)
            .mappedBy((typeSystem, record) -> new Projection() {
              @Override
              public Team getTeam() {
                  return record.get... // how to access single object?
              }

              @Override
              public List<Retailers> getRetailers() {
                  return record.get... // how to access nested list objects?
              }
            })
            .all();
}

The result is a list of following objects:

{
  "drivers": [
    {
      "driver": {
        "name": "Mike",
        "driverId": "15273c10"
      },
      "cars": [
        {
          "carId": "f4ca4581",
          "name": "green car"
        },
        {
          "carId": "11f3bcae",
          "name": "red car"
        }
      ]
    }
  ],
  "team": {
    "teamId": "4586b33f",
    "name": "Blue Racing Team"
  }
}

The problem is now, how to map the response into an according Java model. I don't use the entity classes.
I tried multi-level projection with nested interfaces.

public interface Projection {

    Team getTeam();
    List<Drivers> getDrivers();

    interface Drivers {
        Driver getDriver();
        List<Cars> getCars();
    }

    interface Driver {
        String getDriverId();
        String getName();
    }

    interface Car {
        String getCarId();
        String getName();
    }

    interface Team {
        String getTeamId();
        String getName();
    }
}

I struggle to access the nested lists and objects, to put them into the model.
SDN is the Spring Boot Starter in version 2.6.3.
An example how to map a nested object in a list would be a good starting point.
Or may be my approach is totally wrong? Any help is appreciated.

Answer 1

Projection are not meant to be something like a view or wrapper of arbitrary data.

Within the context you can get a Neo4jMappingContext instance. You can use this to obtain the mapping function for an already existing entity. With this, you do not have to take care about mapping the Car and (partially because of the team relationship) the Drivers .

BiFunction<TypeSystem, MapAccessor, Car> mappingFunction = neo4jMappingContext.getRequiredMappingFunctionFor(Car.class);

The mapping function accepts an object of type MapAccessor . This is a Neo4j Java driver type that is implemented besides others also by Node and MapValue .

You can use those values from your result eg drivers in a loop (should be possible to call asList on the record) and within this loop you would also assign the cars .

Of course using the mapping function would only make sense if you have a lot more properties to map because nothing in the return structure (as you already said between the lines) applies to the entity structure regarding the relationships.

Here is an example of using the mapping function and direct mapping. You have to decide what matches best for your use case.

public Collection<Projection> loadHierarchy() {
    var teamMappingFunction = mappingContext.getRequiredMappingFunctionFor(Team.class);
    var driverMappingFunction = mappingContext.getRequiredMappingFunctionFor(Driver.class);
    return neo4jClient.query("""
            MATCH(t:Team)<--(d:Driver)<--(c:Car)
            WITH t, d, collect(distinct c{.carId, .name}) AS carsEachDriver
            WITH t, collect({driver: d{.driverId, .name}, cars: carsEachDriver }) AS driverEachTeam
            WITH {team: t{.teamId, .name}, drivers: driverEachTeam } as team
            RETURN team
         """)
            .fetchAs(Projection.class)
            .mappedBy((typeSystem, record) -> {
                Team team = teamMappingFunction.apply(typeSystem, record.get("team"));
                List<DriverWithCars> drivers = record.get("team").get("drivers").asList(value -> {
                    var driver = driverMappingFunction.apply(typeSystem, value);
                    var cars = value.get("carsEachDriver").asList(carValue -> {
                        return new Car(value.get("name").asString());
                    });
                    
                    return new DriverWithCars(driver, cars); // create wrapper object incl. cars
                });
                return new Projection(team, drivers);
            })
            .all();
}

(Disclaimer: I did not execute this on a data set, so there might be typos or wrong access to the record) Please note that I changed the cypher statement a little bit to have get one Team per record instead of the whole list. Maybe this is already what you have asked for.