使用JPA映射多对多关系与属性 [英] Mapping many-to-many relationship with attributes with JPA

问题描述

考虑以下实体 - 关系图

  [PERSON]（* .. N）------<< ;参与> ------（* .. N）[活动] 
 / \ 
爱好义务
  

其中 [PERSON] 和 [ACTIVITY] 是实体， < />< / code>与属性（标志） hobby 和 duty 。这些属性是非排他性的：两者可以是相互独立的真或假。

如何使用JPA（或Hibernate）将此ERD映射到Java对象模型？

  @Entity 
 class Person {
 Collection< Activity>爱好; //只有hobby = true的活动
 Collection< Activity>职务; //只有值=真的
} 
 
 
 @Entity 
类活动{
 Collection< Person>爱好者; //只有hobby = true的人
 Collection< Person>专业人士; //只有人的职责=真正
} 
  

解决方案

<

我在JPA 2.1 / Hibernate 5.1上为我工作。

• Person实体ID = PERSON_ID


• 活动ID实体= ACTIVITY_ID


• 连结表名称= PERSON_ACTIVITY >
  
  

    @Entity 
   class Person {
   
   @ManyToMany 
   @JoinTable（name = PERSON_ACTIVITY，
   joinColumns = @ JoinColumn（name =PERSON_ID），
   inverseJoinColumns = @ JoinColumn（name =ACTIVITY_ID））
   @WhereJoinTable（clause =hobby = 1） 
   Set< Activity> hobbyActivities; 
   $ b @ManyToMany（fetch = FetchType.LAZY）
   @JoinTable（name =PERSON_ACTIVITY，
   joinColumns = @ JoinColumn（name =PERSON_ID），
   inverseJoinColumns = @ JoinColumn（name =ACTIVITY_ID））
   @WhereJoinTable（clause =duty = 1）
   Set< Activity> dutyActivities; 
   
   $ b  

  活动实体

    @Entity 
   class Activity {
   
   @ManyToMany 
   @JoinTable（name = PERSON_ACTIVITY，
   joinColumns = @ JoinColumn（name =ACTIVITY_ID），
   inverseJoinColumns = @ JoinColumn（name =PERSON_ID））
   @WhereJoinTable（clause =hobby = 1 ）
   Set< Person> hobbyPeople; 
   $ b @ManyToMany（fetch = FetchType.LAZY）
   @JoinTable（name =PERSON_ACTIVITY，
   joinColumns = @ JoinColumn（name =ACTIVITY_ID），
   inverseJoinColumns = @ JoinColumn（name =PERSON_ID））
   @WhereJoinTable（clause =duty = 1）
   Set< Person> dutyPeople; 
  } 
    

  关键点：

  
  
  
  
  

  • 集合必须在所有情况下显式设置Set

  
  

  • 每个实体中的@ManyToMany集合中的一个必须被标记为延迟加载

  
  

  • 在进行一些测试时，我发现我必须使用DISTINCT很多次不要得到重复的结果（但这只是我的情况）。

  
  
  
  

  Consider the following entity-relationship diagram

  [PERSON] (*..N) ------ < is engaged into > ------ (*..N) [ACTIVITY]
                            /          \
                         hobby        duty
  

  where [PERSON] and [ACTIVITY] are entities and <is engaged into> is a many-to-many relationship with attributes (flags) hobby and duty. The attributes are non-exclusive: both can be true or false independent of each other.

  How would one map this ERD to the Java object model shown below using JPA (or Hibernate)?

  @Entity
  class Person {
      Collection<Activity> hobbies;     // only activities where hobby=true
      Collection<Activity> duties;      // only activities where duty=true
  }
  
  
  @Entity
  class Activity{
      Collection<Person> aficionados;   // only people where hobby=true
      Collection<Person> professionals; // only people where duty=true
  }
  

  解决方案

  Really interesting case.

  I made it work for me on JPA 2.1 / Hibernate 5.1.

  Assuming that:

  • Id of Person entity = PERSON_ID
  • Id of Activity entity = ACTIVITY_ID
  • Name of linking table = PERSON_ACTIVITY

  Person entity:

  @Entity
  class Person {
  
      @ManyToMany
      @JoinTable(name="PERSON_ACTIVITY",
              joinColumns=@JoinColumn(name="PERSON_ID"),
              inverseJoinColumns=@JoinColumn(name="ACTIVITY_ID"))
      @WhereJoinTable(clause = "hobby = 1")
      Set<Activity>  hobbyActivities;
  
      @ManyToMany(fetch = FetchType.LAZY)
      @JoinTable(name="PERSON_ACTIVITY",
              joinColumns=@JoinColumn(name="PERSON_ID"),
              inverseJoinColumns=@JoinColumn(name="ACTIVITY_ID"))
      @WhereJoinTable(clause = "duty = 1")
      Set<Activity>  dutyActivities;
  
  }
  

  Activity entity:

  @Entity
  class Activity{
  
      @ManyToMany
      @JoinTable(name="PERSON_ACTIVITY",
              joinColumns=@JoinColumn(name="ACTIVITY_ID"),
              inverseJoinColumns=@JoinColumn(name="PERSON_ID"))
      @WhereJoinTable(clause = "hobby = 1")
      Set<Person>  hobbyPeople;
  
      @ManyToMany(fetch = FetchType.LAZY)
      @JoinTable(name="PERSON_ACTIVITY",
              joinColumns=@JoinColumn(name="ACTIVITY_ID"),
              inverseJoinColumns=@JoinColumn(name="PERSON_ID"))
      @WhereJoinTable(clause = "duty = 1")
      Set<Person>  dutyPeople;
  }
  

  Key points:

  • The collection has to be explicitly a Set in all of the cases

  • One of the @ManyToMany sets in each entity has to be marked as lazy loaded

  • While doing some testing i found that i had to use DISTINCT a lot of times not to get repeating results (but that was only maybe my case).

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