JPA JoinColumn vs mappedBy [英] JPA JoinColumn vs mappedBy

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>分支机构; 
 ... 
} 
  

和

  @Entity 
 public class Company {
 
 @OneToMany（cascade = CascadeType.ALL，fetch = FetchType.LAZY，mappedBy =companyIdRef）
私人列表< Branch>分支机构; 
 ... 
} 
  

解决方案

@JoinColumn 可以用于关系的两边。问题是关于使用 @JoinColumn 放在 @OneToMany 一边（罕见的情况）。这里的重点在于物理信息复制（列名称）以及未优化的SQL查询，它将产生一些额外的UPDATE语句。

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，因此一对多关联会注释by @OneToMany（mappedBy = ...）

  @Entity 
公开课部队{
 @OneToMany （mappedBy =troop）
 public Set< Soldier> getSoldiers（）{
 ... 
} 
 
 @实体
公共类士兵{
 @ManyToOne 
 @JoinColumn（name = ）
 public Troop getTroop（）{
 ... 
} 
  

部队通过部队财产与士兵有双向一对多的关系。您不必（不得）在mappedBy方中定义任何物理映射。

要使用一对一映射一个双向多对象作为拥有者的一方，您必须移除mappedBy元素，并将多个@JoinColumn设置为可插入和可更新为false。

  @Entity 
 public class Troop {
 @OneToMany 
 @JoinColumn（name =troop_fk）//我们需要复制物理信息
 public Set< Soldier> getSoldiers（）{
 ... 
} 
 
 @实体
公共类士兵{
 @ManyToOne 
 @JoinColumn（name = （），$ {
 
} 
  

What is the difference between:

@Entity
public class Company {

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

and

@Entity
public class Company {

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

解决方案

@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.

According to documentation:

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() {
    ...
} 

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.

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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