JPA多对二关系

Question

I'm new to JPA and was wondering if JPA contains a solution for my problem. Els I will just need to create a ManyToMany relationship.

My application contains roads and camera's. A road starts and ends with a camera. I created this by creating a property cameraPointA and cameraPointB in the RoadSegment class. This created a many to two relationship. I thought I could define this as two many to one relationships but this seems not possible.

CameraPoint.java

@Entity
public class CameraPoint implements Serializable {

    @Id @GeneratedValue
    private long id;

    @OneToMany (mappedBy = "cameraPointA or cameraPointA") //<== The Problem
    private List<RoadSegment> roads;

    //...
}

RoadSegment.java

@Entity
public class RoadSegment implements Serializable {

    @Id @GeneratedValue
    private long id;

    @ManyToOne(cascade = CascadeType.ALL)
    private Region region;

    @ManyToOne(optional=false)
    private CameraPoint cameraPointA;
    @ManyToOne(optional=false)
    private CameraPoint cameraPointB;

    //...
}

Answer 1

I don't know if this will work but maybe you can try adding to CameraPoint another list of RoadSegments , which indicate the Camera has a list of roads where the camera is the start and the other list indicates the camera is the last.

@Entity
public class CameraPoint implements Serializable {

@Id @GeneratedValue
private long id;

@OneToMany (mappedBy = "cameraPointA")
private List<RoadSegment> roadsA;

@OneToMany (mappedBy = "cameraPointB")
private List<RoadSegment> roadsB;

//...
}

Is really necessary to use bidirectional relationship ? Maybe it is not and your model would be more easy.

For example, if you always reach to the CameraPoint from a RoadSegment then you don't need the @OneToMany relationship on CameraPoint .

The same applies for the inverse mode, if you always get a RoadSegment from a previous CameraPoint , then the @ManyToOne relationship is not necessary.