使用JPA @OneToMany关联时@JoinColumn和mappedBy之间有什么区别 [英] What's the difference between @JoinColumn and mappedBy when using a JPA @OneToMany association

问题描述

有什么区别：

@Entity
public class Company {

    @OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY)
    @JoinColumn(name = "companyIdRef", referencedColumnName = "companyId")
    private List<Branch> branches;
    ...
}

和

@Entity
public class Company {

    @OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY, mappedBy = "companyIdRef")
    private List<Branch> branches;
    ...
}

推荐答案

@JoinColumn 可用于关系的双方。问题是关于使用 @JoinColumn 在 @OneToMany 一侧（极少数情况下）。这里的重点是物理信息重复（列名）以及未优化的SQL查询，它将产生一些额外的UPDATE语句。

@JoinColumn could be used on both sides of the relationship. The question was about using @JoinColumn on the @OneToMany side (rare case). And the point here is in physical information duplication (column name) along with not optimized SQL query that will produce some additional UPDATE statements.

根据文档：

由于多对一（几乎）始终是JPA规范中双向关系的所有者方，因此会对一对多关联进行注释by @OneToMany（mappedBy = ...）

Since many to one are (almost) always the owner side of a bidirectional relationship in the JPA spec, the one to many association is annotated by @OneToMany(mappedBy=...)

@Entity
public class Troop {
    @OneToMany(mappedBy="troop")
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne
    @JoinColumn(name="troop_fk")
    public Troop getTroop() {
    ...
} 

部队通过部队属性与士兵有一对一的双向关系。您不必（不得）在mappedBy端定义任何物理映射。

Troop has a bidirectional one to many relationship with Soldier through the troop property. You don't have to (must not) define any physical mapping in the mappedBy side.

要将双向的映射到多个，使用 one-to - 很多方面作为拥有方，您必须删除mappedBy元素并将多个@JoinColumn设置为可插入且可更新为false。此解决方案未经过优化，将生成一些额外的UPDATE语句。

To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the mappedBy element and set the many to one @JoinColumn as insertable and updatable to false. This solution is not optimized and will produce some additional UPDATE statements.

@Entity
public class Troop {
    @OneToMany
    @JoinColumn(name="troop_fk") //we need to duplicate the physical information
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne
    @JoinColumn(name="troop_fk", insertable=false, updatable=false)
    public Troop getTroop() {
    ...
}

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