如何在JPA父实体中将子关联属性用作Map键

Question

I'm having two entities Car and CarDescription where CarDescription is depending on another foreign key from the table Language .

What I' trying to accomplish is to have a HashMap in Car such that whenever I'm having a Car entity-object I am able to access all descriptions from the language id.

Entity 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);
}

Entity 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;

    // ..
}

Entity 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;

    // ..
}

The problem I am having is that the mapping gives me a map from each CarDescription.id to CarDescription .

How can I accomplish a correct mapping?

Answer 1

In CarDescription you need to add the languageId property:

@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;
} 

Then you can use it in the Car entity like this:

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