使用@ManyToMany 注释从连接表中级联删除 [英] Cascade deleting from join table with @ManyToMany annotation

问题描述

我在映射实体时遇到问题.我正在使用 JPA2 和 Hibernate 实现.我得到了带有@ManyToMany 注释的表格

Hi I got a problem with mapping my entities. I'm using JPA2 and Hibernate implementation. I got tables with @ManyToMany annotation

http://img204.imageshack.us/img204/7558/przykladd.png

我用:

@Entity
@Table("employee")
class Employee {
      @Id
      @GeneratedValue(strategy = GenerationType.IDENTITY)
      private Integer id;

  @Column
  private String name; 

  @ManyToMany
  @JoinTable(name = "proj_emp",
             joinColumns = @JoinColumn(name = "employee_id", referencedColumnName = "id"),
             inverseJoinColumns = @JoinColumn(name = "project_id", referencedColumnName = "id"), 
             uniqueConstraints = @UniqueConstraint(columnNames = {"employee_id", "project_id"})) 
  private List<Project> projects;                ...}


@Entity
@Table("project")
class Project {
   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   private Integer id; 

   @Column  
   private String name;    
      
   @Column    
   private Integer budget;        

   @ManyToMany(mappedBy = "projects")     
   private List<Employee> employees;                ...}

现在，当我从 Employee 中删除记录时，我想从表 proj_emp 中进行级联删除，但无法删除表 Project 中的任何内容.

Now I would like to have a cascade deleting from table proj_emp when I delete records from Employee, but nothing from table Project can be deleted.

获得它的最佳方式是什么?

What is the best way to acquire that?

谢谢大卫

推荐答案

您可以将@ManyToMany 拆分为@OneToMany-ManyToOne 并设置级联样式，如图所示here 虽然问题使用的是Hibernate的会话，但您可以使用JPA EntityManager.或者使用新的 JPA 功能 @ElementCollection(仅限JPA 2) 映射您加入的类.请参阅此处如何操作.只需将 Hibernate 的 @CollectionOfElements 替换为 @ElementCollection

You can split your @ManyToMany into a @OneToMany-ManyToOne and set up a cascading style as shown here Although the question uses Hibernate's session, you can use JPA EntityManager. Or use the new JPA feature @ElementCollection (Only JPA 2) to map your joined class. See here how to. Just replace Hibernate's @CollectionOfElements by @ElementCollection

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