在JPA / Hibernate中建模多对多关系

问题描述

架构在这里：

或这里;

现在，我创建了以下类：

1. Student.java

     @Entity 
    public class Student实现Serializable {
    
    @Id 
    @GeneratedValue 
    private int id; 
    
    @NotNull 
    @Size（min = 1，max = 35）
   私人字符串名称; 
    
    public int getId（）{
    return id; 
   } 
    
    public void setId（int id）{
    this.id = id; 
   } 
    
    public String getName（）{
    return name; 
   } 
    
    public void setName（String name）{
    this.name = name; 
   } 
   } 
     




2. Friend.java

     @Entity 
    public class Friend实现Serializable {
    
    @EmbeddedId 
    @NotNull 
    @ManyToMany（targetEntity = Student.class）
    private FriendPK primaryKey; 
    
    
    public FriendPK getPrimaryKey（）{
    return primaryKey; 
   } 
    
    public void setPrimaryKey（FriendPK primaryKey）{
    this.primaryKey = primaryKey; 
   } 
   } 
     




3. FriendPK.java

   
   
   'pre> @Embeddable
   公共类FriendPK实现Serializable {
   
   @JoinColumn（名称= id_from，referencedColumnName = id）
   private int idFrom;
   
   @JoinColumn（name =id_to，referencedColumnName =id）
   private int idTo;
   $ b $ public FriendPK（）{
   }
   
   public int getIdFrom（）{
   return idFrom;
   }
   
   public void setIdFrom（int idFrom）{
   this.idFrom = idFrom;
   }
   
   public int getIdTo（）{
   return idTo;
   }
   
   public void setIdTo（int idTo）{
   this.idTo = idTo;
   
   
   code $
   
   
   

   1）这足以模拟多对多的关系吗？更具体地说，
   
   我是否需要Student类中的另一个List friends变量来存储这个学生的朋友？ 3）同样，在Friend类中是否需要另一个List students变量来存储朋友的学生？

   
   
   我隐约感到目前的设计还不够，但无法弄清楚需要什么。
   
   
   

   谢谢。

   解决方案

   你只需要一个持久化来实现这样的模式。 Hibernate会为 @ManyToMany 关联创建一个连接表。如果您需要指定连接表的名称，则可以使用 @JoinTable 注释。

   
   
   

   需要 Friend 类。你可以用 HQL 或 Criteria 来处理 friends 。所以你不需要知道连接表。在将 HQL 转换为 SQL 时，Hibernate将为连接表添加连接。

   
   
   

   from Student's inner join fetch s.friends where s.name =：studentName

   
   
   

   Keep记住它是 HQL ，对于 JPQL 您需要选择
   
   

     @Table 
    @Entity 
   公共班学生{
    
    @I $ 
    @GeneratedValue 
    private int id; 
    
    @NotNull 
    @Size（min = 1，max = 35）
   私人字符串名称; 
    
    @ManyToMany 
   私人列表< Student> friends = new ArrayList< Student>（）; 
    
    public int getId（）{
    return id; 
   } 
    
    public void setId（int id）{
    this.id = id; 
   } 
    
    public String getName（）{
    return name; 
   } 
    
    public void setName（String name）{
    this.name = name; 
   } 
    
   公共列表< Student> getFriends（）{
   返回朋友; 
   } 
    
    public void setFriends（List< Student> friends）{
    this.friends = friends; 
   } 
    
   } 
     

   
   

   The schema is here:http://sqlfiddle.com/#!9/5ec63/2

   or here;Query from many-to-many relationship

   Now, I created the following classes:

   1. Student.java

      @Entity
      public class Student implements Serializable {
      
      @Id
      @GeneratedValue
      private int id;
      
      @NotNull
      @Size(min = 1, max = 35)
      private String name;
      
      public int getId() {
          return id;
      }
      
      public void setId(int id) {
          this.id = id;
      }
      
      public String getName() {
          return name;
      }
      
      public void setName(String name) {
          this.name = name;
      }
      }
      

   2. Friend.java

      @Entity
      public class Friend implements Serializable {
      
      @EmbeddedId
      @NotNull
      @ManyToMany( targetEntity = Student.class )
      private FriendPK primaryKey;
      
      
      public FriendPK getPrimaryKey() {
          return primaryKey;
      }
      
      public void setPrimaryKey(FriendPK primaryKey) {
          this.primaryKey = primaryKey;
      }
      }
      

   3. FriendPK.java

      @Embeddable
      public class FriendPK implements Serializable {
      
      @JoinColumn( name = "id_from", referencedColumnName = "id")
      private int idFrom;
      
      @JoinColumn( name = "id_to", referencedColumnName = "id")
      private int idTo;
      
      public FriendPK() {
      }
      
      public int getIdFrom() {
          return idFrom;
      }
      
      public void setIdFrom(int idFrom) {
          this.idFrom = idFrom;
      }
      
      public int getIdTo() {
          return idTo;
      }
      
      public void setIdTo(int idTo) {
              this.idTo = idTo;
          }
         }
      

   1) Is this enough to model the many-to-many relationship? More specifically,

   2) Do I need another "List friends" variable in Student class to store this student's friends?

   3) Likewise, do I need another "List students" variable in Friend class to store students which are friends?

   I vaguely feel the current design isn't enough, but can't figure out exactly what's needed.

   Thank you.

   解决方案

   You need only one persistent to implement such schema. Hibernate will create a join table for @ManyToMany association. If you need to specify name of the join table you can use a @JoinTable annotation.

   You don't need the Friend class. You can deal with friends with HQL or Criteria. So you don't need to know about a join table. Hibernate will add joins for a join table while converting HQL to SQL.

   from Student s inner join fetch s.friends where s.name = :studentName

   Keep in mind that it is HQL, for JPQL you need select.

   @Table
   @Entity
   public class Student {
   
       @Id
       @GeneratedValue
       private int id;
   
       @NotNull
       @Size(min = 1, max = 35)
       private String name;
   
       @ManyToMany
       private List<Student> friends = new ArrayList<Student>();
   
       public int getId() {
           return id;
       }
   
       public void setId(int id) {
           this.id = id;
       }
   
       public String getName() {
           return name;
       }
   
       public void setName(String name) {
           this.name = name;
       }
   
       public List<Student> getFriends() {
           return friends;
       }
   
       public void setFriends(List<Student> friends) {
           this.friends = friends;
       }
   
   }
   

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