具有继承的密钥和一对多关系的JPA实体：无法获取适当的外键

Question

I have 3 entities:

1. a Poi entity
2. a PolygonPoi entity that inherits from the Poi entity (also the @Id) via a joined inheritance type.
3. and the Coordinate entity that is connected to the Polygon poi via a many to on relation (1-PolygonPoi N-Coordinates)

My problem is that I can't get proper foreign keys ( I get only only null values) for the Coordinate entity. Coordinate should use the Id of Poi. How do I fix this ??

Here are the three mentioned entity classes:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Poi
{
    @Id
    @Column(name = "NAME")
    protected String name;

---

@Entity
public class PolygonPoi extends Poi
{
    @OneToMany(mappedBy = "poi", cascade={CascadeType.ALL})
    protected List<Coordinate> coordinate = new ArrayList<Coordinate>();

---

@Entity
public class Coordinate
{
    @Id
    @GeneratedValue
    private long id;

    @ManyToOne  
    Poi poi;

Also It would be nice to get ride of PolygonPoi in the Database entirely, since it does not hold any information beside the relation to the Coordinate.

E:

Here is how my Tables look like:

SELECT * FROM POI;
NAME    CREATEDBY  
pp0     1

SELECT * FROM POLYGONPOI;
NAME  
pp0

SELECT * FROM COORDINATE ;
ID      LATITUDE            LONGITUDE           POI_NAME  
1       |30.030000686645508 |50.29999923706055  |null
2       |31.030000686645508 |51.29999923706055  |null
3       |31.030000686645508 |50.29999923706055  |null

Answer 1

Uunless you specify a @JoinColumn annotation and define the foreign key column, JPA will default to using a "poi_NAME" field within the Coordinate table as a foriegn key to Poi.NAME for the Coordinate ->Poi relationship. This will be set with the name value contained within the referenced poi instance. If the referenced poi doesn't have a name set - your application is responsible for assigning it as a Poi entity (or subclass) cannot exist without an ID.