从级联删除连接表与@ManyToMany注解 [英] Cascade deleting from join table with @ManyToMany annotation

问题描述

您好我有一个问题，我的映射实体。我使用JPA2和Hibernate实现。我的表与@ManyToMany注释

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

我映射的：

@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;                ...}

现在我想有一个级联从表中删除proj_emp当我从员工中删除记录，但是从表项目没有什么可以被删除。

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.

什么是收购的最佳途径？

What is the best way to acquire that?

谢谢
的Dawid

Thanks Dawid

推荐答案

您可以将您的@ManyToMany分成@一对多，多对一，并成立了级联样式如图所示的此处虽然问题使用Hibernate的会话，你可以使用JPA的EntityManager。或者使用新的JPA功能 @ElementCollection （仅JPA 2）来映射你的加入类。请参见这里如何。仅仅通过更换@ElementCollection Hibernate的@CollectionOfElements

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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