JPA JoinColumn 与 mappingBy [英] JPA JoinColumn vs mappedBy

问题描述

有什么区别:

@Entity公共类公司{@OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY)@JoinColumn(name = "companyIdRef", referencedColumnName = "companyId")私人列表<分支>分支机构；...}

和

@Entity公共类公司{@OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY,mappingBy = "companyIdRef")私人列表<分支>分支机构；...}

解决方案

@JoinColumn 可以在关系的双方使用. 问题是关于使用 @JoinColumn 在 @OneToMany 端(罕见情况).这里的重点在于物理信息重复(列名)以及未优化的 SQL 查询，这些查询将产生一些额外的 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，因此一对多关联由@注释OneToMany(mappedBy=...)

@Entity公共课部队{@OneToMany(mappedBy="部队")公共集<士兵>获取士兵(){...}@实体公开课士兵{@ManyToOne@JoinColumn(name="troop_fk")公共部队 getTroop() {...}

Troop 通过 troop 属性与 Soldier 存在双向一对多关系.您不必(不得)在 mappedBy 端定义任何物理映射.

要映射双向一对多，以一对多作为拥有方，您必须删除mappedBy元素并将多设置为一个 @JoinColumn 作为 insertable 和 updatable 为假.此解决方案未优化，将生成一些额外的 UPDATE 语句.

@Entity公共课部队{@一对多@JoinColumn(name="troop_fk")//我们需要复制物理信息公共集<士兵>获取士兵(){...}@实体公开课士兵{@ManyToOne@JoinColumn(name="troop_fk"，可插入=假，可更新=假)公共部队 getTroop() {...}

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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