复合主键导致 org.hibernate.NonUniqueObjectException

Question

我正在尝试为表 City 使用复合主键，如果它没有分配给 Country，则该表不应该存在（它应该是多对一 识别关系）。

我的问题首先是ChildId（参见嵌套类）在我初始化Child 之后有一个@GeneratedValue 即null，如下所示：

List<City> cityList = new ArrayList<>();    
City graz = new City(austria, "Graz");
// Runs without exceptions but graz.getCityId().getId() will be 'null'
cityList.add(graz);
cityList.add(new City(austria, "Wien"));

似乎因为它我得到了一个org.hibernate.NonUniqueObjectException 异常：

完整的堆栈跟踪：

Exception in thread "main" org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [com.mahlzeit.datamodel.address.City#com.mahlzeit.datamodel.address.City$CityId@8a62f66]
    at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:182)
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:138)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:204)
    at org.hibernate.event.internal.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:55)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:189)
    at org.hibernate.event.internal.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:49)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:90)
    at org.hibernate.internal.SessionImpl.fireSave(SessionImpl.java:642)
    at org.hibernate.internal.SessionImpl.save(SessionImpl.java:635)
    at org.hibernate.internal.SessionImpl.save(SessionImpl.java:631)
    at com.mahlzeit.datamodel.HibernateTest.addAddressData(HibernateTest.java:63)
    at com.mahlzeit.datamodel.HibernateTest.populateDatabase(HibernateTest.java:35)
    at com.mahlzeit.datamodel.HibernateTest.main(HibernateTest.java:31)

我不知道我是否可以这样做。如果是，我怎样才能做到这一点？

City.java

@Entity
public class City implements Serializable {

    private static final long serialVersionUID = -4374113410767348574L;

    @EmbeddedId
    private CityId cityId;

    private String cityName;

    public City(Country country, String cityName) {
        setCityId(new CityId(country));
        this.cityName = cityName;
    }

    public String getCityName() {
        return cityName;
    }

    public void setCityName(String countryName) {
        this.cityName = countryName;
    }

    public CityId getCityId() {
        return cityId;
    }

    public void setCityId(CityId cityId) {
        this.cityId = cityId;
    }

    @Embeddable
    public static class CityId implements Serializable{

        private static final long serialVersionUID = -8561021314776406519L;

        @GeneratedValue
        private Long id;

        private Country country;

        public CityId(Country country) {
            this.setCountry(country);
        }

        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        public Country getCountry() {
            return country;
        }

        public void setCountry(Country country) {
            this.country = country;
        }       
    }
}

Country.java

@Entity
public class Country implements Serializable {

    private static final long serialVersionUID = -2060021861139912774L;

    @Id 
    @GeneratedValue
    private Long id;

    @Column(unique=true)
    private String countryCode;

    public Country(String country_code) {
        this.countryCode = country_code;
    }

    public String getCountryCode() {
        return countryCode;
    }

    public void setCountryCode(String countryCode) { 
        this.countryCode = countryCode;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }
}

Answer 1

对于City 类，主键是CityId，并且这个类有两个属性——id 和country。

请注意，@GeneratedValue 只有在属性也用@Id 注释时才有用，因此在CityId 类中为id 属性添加GeneratedValue 没有用处，休眠将忽略这一点。你也不能为这个属性添加@Id，因为这个类代表@Embeddable

现在在您的程序中，您正在像这样设置CityId：

public City(Country country, String cityName) {
    setCityId(new CityId(country));
    this.cityName = cityName;
}

CityId的构造函数是：

    public CityId(Country country) {
        this.setCountry(country);
    }

所以最后你要创建 2 个具有相同国家/地区的城市：

City graz = new City(austria, "Graz");
new City(austria, "Wien");

此外，由于您没有为 CityId 类的 id 设置值，因此它将始终为 null。因此city 对象graz 和wien 具有相同的主键，因为country 引用和id 的组合（在这种情况下为空）。

---

您还需要在实体之间进行适当的映射，请参阅此 SO 帖子以获取类似示例：

How to Represent Composite keys in Hibernate using Annotations?

来到CityId 的id 属性，您必须明确设置它，否则它将始终为null。