方法一：
@Entity
@IdClass(ModuleId.class)
public class Module {

    @Id
    private Integer index;

    @Id
    @ManyToOne
    private Page page;

    private Integer cols;
}

方法二：

@Entity
public class Element {

    @EmbeddedId
    private ElementId id;

    @ManyToOne
    @MapsId("pageId")
    @JoinColumn(name = "mypage_id")
    private Page page;

    private String name;
}

JPA 派生標識符的兩種實現方式