如何在JPA父實體中將子關聯屬性用作Map鍵

Question

我有兩個實體Car和CarDescription ，其中CarDescription依賴於表Language另一個外鍵。

我想要實現的是在Car有一個HashMap ，這樣每當我有一個Car實體對象時，我就可以從語言id訪問所有描述。

實體Car.java

@Entity
@Table(name = "Car")
public class Car extends AbstractTimestampEntity implements Serializable {
    private static final long serialVersionUID = -5041816842632017838L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Long id;

    @OneToMany(mappedBy="car")
    @MapKeyColumn(name = "language_ID")
    // @MapKey(name = "language") // does not work either 
    private Map<Long, CarDescription> carDescription = new HashMap<>(0);
}

實體CarDescription.java

@Entity
@Table( name="car_description",
        uniqueConstraints = {
            @UniqueConstraint(columnNames={"language_id", "name"}) 
        }
)
public class CarDescription extends AbstractTimestampEntity implements Serializable {
    private static final long serialVersionUID = 2840651722666001938L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Long id;

    @NotNull
    @ManyToOne
    private Car car;

    @NotNull
    @OneToOne
    private Language language;

    // ..
}

實體Language.java

@Entity
public class Language implements Serializable {
    private static final long serialVersionUID = 3968717758435500381L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="ID")
    private Long id;

    // ..
}

我遇到的問題是映射給我一個從CarDescription.id到CarDescription 。

我怎樣才能完成正確的映射？

Answer 1

在CarDescription您需要添加languageId屬性：

@Column(name = "language_id", insertable = false, updatable = false)
private Long languageId;

@NotNull
@OneToOne
@JoinColumn(name = "language_id")
private Language language;

public void setLanguage(Language language) {
    this.languageId = language.getId();
    this.language = language;
} 

然后你可以在Car實體中使用它，如下所示：

@OneToMany(mappedBy="car")
@MapKey(name = "languageId")
private Map<Long, CarDescription> carDescription = new HashMap<>(0);