JPA + Hibernate - 如何获取子实体的FK而无需获取该实体？ [英] JPA + Hibernate - How to get FK of child entity without fetching that entity?

问题描述

我的问题可能的答案位于这里：
如何从JPA ManyToOne映射中检索外键而无需触击目标表？

然而，更好的解决方案（属性访问）在我的情况下不起作用（我缺少列异常 - 为什么？）

模型如下所示：实体父和小孩。表父的列 child_id ， PK of child 表，所以它通常是 @ManyToOne 关系。

现在问题是，如果我获取父实体，我需要访问 FK 值（aka。<$ c （code> Child entity）的$ c> PK 没有获取 Child 实体。我怎么做？

我使用注解，我的映射如下所示：

  @Entity 
 @Table（name =parent）
 public class Parent extends AbstractEntity {
 
 @ManyToOne（fetch = FetchType.LAZY）
 @JoinColumn（name =patent_id，nullable = true）
私人子女; 
 
 @Column（name =child_id，insertable = false，updatable = false）
 private Integer childId; 
 
 public Child getChild（）{
 return return patent; 
} 
 
 public void setChild（Child child）{
 this.child = child; 
} 
 
 
 public Integer getChildId（）{
 return childId; 
} 
} 
  

我想要做的是调用 parent.getChild（）。getId（）不需要从数据库中额外提取 Child 实体。

根据我上面提到的答案，如果我将注释从字段移到getter（在父实体中，我是否正确？），所请求的行为将是盒子外面。然而，当我将注释移动到getter时，我得到一个验证异常，它缺少 child 列（好奇，为什么 child 而不是 child_id ）声明？）

PS： c $ c> FK 列作为单独的字段可以正常工作，但我不认为这是应该完成的。

解决方案

好了，阅读以下文章 http：// 256stuff .com / gray / docs / misc / hibernate_lazy_field_access_annotations.shtml
我已经意识到，该属性访问应该是我要获取的属性，而不是实际的子对象。因此改变 id 从到的 AbstractEntity 访问权限，就可以达到目的。

A possible answer to my question is located here: How can I retrieve the foreign key from a JPA ManyToOne mapping without hitting the target table?

However, the preferable solution (property access) does not work in my case (I got missing column exception - why?)

The model looks like this: Entities Parent and Child. Table parent has column child_id which is PK of child table so it is typical @ManyToOne relation.

Now the point is, if I fetch Parent entities, I need to have access to FK value (aka. PK of Child entity) without fetching Child entities. How can I do that?

I use Annotations and my mapping looks as follows:

@Entity
@Table(name = "parent")
public class Parent extends AbstractEntity {

@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name = "patent_id", nullable = true)
private Child child;

@Column(name="child_id",insertable=false,updatable=false)
private Integer childId;

public Child getChild() {
    return patent;
}

public void setChild(Child child) {
    this.child = child;
}


public Integer getChildId(){
    return childId;
}
}

And what I want to do is call parent.getChild().getId() without extra fetches of Child entity from DB.

According to the answer I have mentioned above, If I moved annotations from field to getter (in Parent entity am I right?), requested behavior would be out of the box. However, when I move annotations to getter, I get a validation exception that child column is missing (curious, why child not child_id as declared?)

PS: Shown workaround to declare a FK column as separate field works fine, but I don't think that this is the way it should be done.

解决方案

OK, after reading following article http://256stuff.com/gray/docs/misc/hibernate_lazy_field_access_annotations.shtml I have realized, that property access should be to the property I want to fetch, not the actual child object. So changing id access of AbstractEntityfrom field to property, makes the trick.

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