Question

有三个表 A，B，A_B_relation，最后一个是A和B的关联表。

Columns of table A:              id, value_a
Columns of table B:              id, value_b
Columns of table A_B_relation:   id, a_id, b_id

您可以在下面找到A和B的映射类。请注意，A类中有一个字段“B b”。

@Entity
@Table(name = "A")
public class A {

    @GenericGenerator(name = "idGenerator", strategy = "increment")
    @Id
    @GeneratedValue(generator = "idGenerator")
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @Column(name = "value_a")
    private String valueA;

    private B b;
} 

@Entity
@Table(name = "B")
public class B {

    @GenericGenerator(name = "idGenerator", strategy = "increment")
    @Id
    @GeneratedValue(generator = "idGenerator")
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @Column(name = "value_b")
    private String valueB;
}

是否有可能在hibernate中通过session.get("A", id)初始化b来获取实例A？正如我所知，应该有表连接，例如A a left join A_B_relation r on a.id = r.a_id left join B b on r.b_id = b.id，但我不确定如何通过hibernate实现它。

提前致谢。

Answer 1

因为你没有描述A和B之间的关系，所以是OneToOne ManyToOne OneToMany ManyToMany我将假设它是A的一侧的ManyToOne，因为每个A只与一个B相关，你需要第三个表进行连接，使每个A与一个B相关，但B可以与许多A（s）相关

  @Entity
    @Table(name = "A")
    public class A {

        @GenericGenerator(name = "idGenerator", strategy = "increment")
        @Id
        @GeneratedValue(generator = "idGenerator")
        @Column(name = "id", unique = true, nullable = false)
        private Integer id;

        @Column(name = "value_a")
        private String valueA;
        @joinTable(name="A_B_relation",joinColumns=@JoinColumn(name="a_id"),
                   inverseJoinColumns=@JoinColumn(name="b_id"))
        @ManyToMany(fetch=FetchType.EAGER)
        private List<B> b;
    }

通过Hibernate的关联表检索对象？

1 个答案: