通过Hibernate的关联表检索对象？

Question

There are three tables A, B, A_B_relation, and the last one is the associate table of A and B.

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

You may find the mapping classes for A and B as below. Please note that there is a field "B b" in class A.

@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;
}

Is it possible to get instance A with initialized b by session.get("A", id) in hibernate? As I known, there should be tables join such as A a left join A_B_relation r on a.id = r.a_id left join B b on r.b_id = b.id , but I'm not sure how to implement it by hibernate.

Thanks in advance.

Answer 1

since you didn't describe the relation between A and B , is it OneToOne ManyToOne OneToMany ManyToMany i will assume that it is ManyToOne from the side of A. since each A is related to only one B and you needed a third table to make the join so each A is related to one B but B can be related to many A(s)

  @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;
    }