为什么hibernate会为OneToMany映射生成插入和更新 [英] Why hibernate generates insert and update for OneToMany mapping

问题描述

我试图通过一个小例子来理解Hibernate中的一对多映射。我有一个产品，其中一套零件的。以下是我的实体类：
$ b preart c $ Part.java

$ b

  @Entity 
 public class Part {
 @Id 
 @GeneratedValue 
 int id; 
 String partName; 
 // Setters& Getters 
 
  

Product.java

  @Entity 
 public class Product {
 private String serialNumber; 
私人设定< Part> parts = new HashSet< Part>（）; 
 @Id 
 public String getSerialNumber（）{
 return serialNumber; 
} 
 @OneToMany 
 @JoinColumn（name =PRODUCT_ID）
 public Set< Part> getParts（）{
返回零件; 
} 
 // Setter方法
} 
  

然后我尝试在我的数据库中保存一些零件和产品，并观察由hibernate生成的查询： （？）
Hibernate：插入Part（partName，id）值（？，？）
Hibernate：更新Part set PRODUCT_ID =？其中id =？

这里要在零件表中添加记录，hibernate生成2个DML操作 - 插入和 update 。如果一个 insert 命令足以在表中添加记录，那么为什么hibernate在这种情况下同时使用插入和更新？请解释。
解决方案

我知道这是疯狂的旧的，但我有同样的问题，谷歌带我到这里，所以修复后我想我应该发布一个答案。

Hibernate会将插入/更新方法切换为直接插入，如果您使连接列不可空或不可更新，我假设您的情况它不是反正：

  @JoinColumn（name =PRODUCT_ID，nullable = false，updatable = false）
  

I am trying to understand the one-to-many mapping in Hibernate with a small example. I have a Product with a set of Part's. Here are my entity classes:

Part.java

@Entity
public class Part {
    @Id
    @GeneratedValue
    int id;
    String partName;
    //Setters & Getters
}

Product.java

@Entity
public class Product {
    private String serialNumber;
    private Set<Part> parts = new HashSet<Part>();
    @Id
    public String getSerialNumber() {
        return serialNumber;
    }
    @OneToMany
    @JoinColumn(name = "PRODUCT_ID")
    public Set<Part> getParts() {
        return parts;
    }
    // Setter methods
}

Then I tried to save some parts and products in my database and observed below queries generated by hibernate:

Hibernate: insert into Product (serialNumber) values (?)
Hibernate: insert into Part (partName, id) values (?, ?)
Hibernate: update Part set PRODUCT_ID=? where id=?

Here to add a record in Part table, hibernate generates 2 DML operations - insert and update. If a single insert command is sufficient to add a record in table then why hibernate uses both insert and update in this case? Please explain.

解决方案

I know this is crazy old but I had the same problem and Google brought me here, so after fixing it I figured I should post an answer.

Hibernate will switch the insert/update approach to straight inserts if you make the join column not nullable and not updatable, which I assume in your case it is neither anyways:

@JoinColumn(name = "PRODUCT_ID", nullable = false, updatable = false)

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