在Hibernate映射OneToMany关系的路上

Let's take as example two entities A and B. Each A may have many associated Bs. Each B is associated to exactly one A. I represent this relationship with Hibernate as follows:

让我们以两个实体A和B为例。每个A可以有许多相关的B。每个B只与一个A相关联。我用Hibernate表示这种关系,如下所示:

@Entity
public class A {

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

    @Column(name = "AName", nullable = false, unique = true)
    private String name;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "A_B", joinColumns = { @JoinColumn(name = "AId") }, inverseJoinColumns = { @JoinColumn(name = "BId") })
    private Set<B> bs = new HashSet<B>();

}


@Entity
public class B {

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

    @Column(name = "BName")
    private String name;

    @ManyToOne(optional = false)
    @JoinColumn(name = "BA")
    private A a;

}

When I generate the DB from this mapping, here's what I get:

当我从这个映射生成数据库时,这是我得到的:

Table A.
The join table A_B with 2 columns: AId and BId. AId and BId belong to the primary key. And since there is a OneToMany association from A to B (i.e. one A may have many associated Bs, and one B is associated to exactly one A), a unique constraint is added on BId.

连接表A_B有2列:AId和BId。 AId和BId属于主键。并且由于存在从A到B的OneToMany关联(即,一个A可能具有许多关联的B,并且一个B恰好与一个A关联),所以在BId上添加唯一约束。

Table B with columns including BA which is a foreign key referencing column AId of table A.

表B包含BA,其中BA是表A的外键引用列AId。

Concerning the join table,

关于连接表,

why does Hibernate map the OneToMany relationship this way?

为什么Hibernate以这种方式映射OneToMany关系?

It could simply define the primary key as consisting of one column (BId) and the same rules would be enforced.

它可以简单地将主键定义为由一列(BId)组成,并且将强制执行相同的规则。

Is there a reason why the mapping is done this way?

有没有理由以这种方式完成映射?

What are the advantages/drawbacks of such a mapping?

这种映射有哪些优点/缺点?

I precise I'm using Hibernate 4.1.6.Final.

我确切地说我正在使用Hibernate 4.1.6.Final。

1 个解决方案

#1

I see that you are trying to map a bidirectional association: from A to B and from B to A.

我看到你试图映射双向关联:从A到B,从B到A.

Concerning the join table, why does Hibernate map the OneToMany relationship this way?

关于连接表,为什么Hibernate会以这种方式映射OneToMany关系?

Because you specified a join table. You can map a @OneToMany relation without it:

因为您指定了连接表。您可以在没有它的情况下映射@OneToMany关系:

@OneToMany(cascade = CascadeType.ALL)
@JoinColumn(name = "AId", nullable = false)
private Set<B> bs = new HashSet<B>();

or better as you have a bidirectional association:

或者更好,因为你有一个双向关联:

@OneToMany(cascade = CascadeType.ALL, mappedBy = "a")  
private Set<B> bs = new HashSet<B>();

...

@ManyToOne(optional = false)  
@JoinColumn(name = "AId", nullable = false)  
private A a;

Regarding

Is there a reason why the mapping is done this way? What are the advantages/drawbacks of such a mapping?

有没有理由以这种方式完成映射?这种映射有哪些优点/缺点?

The mapping with the join table is useful when you have an optional one to many, if, for example B could have no A. It allows you to avoid a nullable AId column on the B table.

如果您有一个可选的一对多,那么使用连接表的映射非常有用,例如,如果B可能没有A.它允许您避免B表上可以为空的AId列。

#1