使用JPA设置一个对多个桌面的桌面 [英] Setting up a One To ManyJoins Against a Bridge Table using JPA

问题描述

我在MySQL 5数据库中有以下预先存在的表：

• 应用


• 用户


• User_App_Bridge
  
  

  这些关系被映射到App和User之间的User_App_Bridge表（正如名字所暗示的那样，是一个桥梁），作为一对多基数。
  
  ER图：
  
  用户------ User_App_Bridge ------应用程序
  
  p> 1用户与User_App_Bridge表中的许多应用程序相关联。

  
  
  

  1 App与User_App_Bridge表中的许多用户相关联。

  
  
  

  User_App_Bridge表的DDL：
  
  

    CREATE TABLE`User_App_Bridge`（
  `User_App_Bridge_Id` int（11）NOT NULL AUTO_INCREMENT，
  `User_Id` int（11）NOT NULL，
  `App_Id` int（11）NOT NULL，
   PRIMARY KEY（`User_App_Bridge_Id`），
   KEY`App_Id`（`App_Id`），
   KEY`User_Id`（`User_Id`），
   CONSTRAINT`user_app_bridge_ibfk_1` FOREIGN KEY（`App_Id`）REFERENCES`App`（`App_Id`）
  ）ENGINE = InnoDB DEFAULT CHARSET = utf8; 
    

  假设我有以下映射到这些表的JPA注释类：

    @Entity 
  公共类应用程序{
   @Id 
   @GeneratedValue（strategy = GenerationType.AUTO）
   @Column（name =App_Id）
  私人长appId; 
   
   @OneToMany 
   @JoinTable 
  （
   name =UserAppBridge，
   joinColumns = {@JoinColumn（name =App_Id，referencedColumnName = {@JoinColumn（name =User_Id，referencedColumnName =User_Id）} 
  ）
   private List< User>用户; 
   
   @Column（name =App_Name）
   private String appName; 
   
   // Getters& Setter方法
   
   
  实体
  公共类用户
   @Id 
   @GeneratedValue（strategy = GenerationType.AUTO）
   @Column （name =User_Id）
   private long userId; 
   
   @OneToMany 
   @JoinTable 
  （
   name =UserAppBridge，
   joinColumns = {@JoinColumn（name =User_Id，referencedColumnName = User_Id）}，
   inverseJoinColumns = {@JoinColumn（name =App_Id，referencedColumnName =App_Id，unique = true）} 
  ）
   private List< App>应用; 
   
   @Column（name =User_Name）
   private String userName; 
   
   // Getters& Setter方法
   
   $ b $ @Entity 
   public class UserAppBridge {
   
   @Id 
   @GeneratedValue（strategy = GenerationType.AUTO）
   @Column（name =User_App_Bridge_Id）
  私人长userAppBridgeId; 
   
   @Column（name =User_Id）
   private long userId; 
   
   @Column（name =App_Id）
   private long appId; 
   
   // Getters& Setter方法
   
   code $ 
   
   
   
  问题：
   
   
   
   
   
  这是（@JoinTable中的块）是用UserAppBridge为User和App实现一对多映射的正确方法吗？
   
   
   
   
  在@JoinTable中，列&参考列名被分配给SQL值（例如，name =User_Id，referencedColumnName =User_Id）
  或者它应该是Java引用名称（例如name =userId，referencedColumnName =userId）？ 
   
   
  里面的inverseJoinColumns代码为黑色，是unique = true必需的（它是什么）？
   
   
   
   
   
   
   

我需要在UserAppBridge类中做任何其他的事情（用于连接到应用程序和用户）感谢您花时间阅读本文...


解决方案

您的问题的答案：



1.不，它不是。您需要在User和UserAppBridge之间创建oneToMany映射，并在App和UserAppBridge之间创建oneToMany映射。这里是代码：



在用户实体中：

  @OneToMany （mappedBy =user）
 private设置< UserAppBridge> userAppBridgeSet; 
  

在App实体中：



  @OneToMany（mappedBy =app）
 private设置< UserAppBridge> userAppBridgeSet; 
  

在UserAppBridge实体中：



  @Entity 
 @IdClass（UserAppBridgeId.class）
 public class UserAppBridge {
 
 @Id 
 @Column（name =User_App_Bridge_Id ）
私人长userAppBridgeId; 
 
 @Id 
 @Column（name =User_Id）
 private long userId; 
 
 @Id 
 @Column（name =App_Id）
 private long appId; 
 
 @ManyToOne 
 @PrimaryKeyJoinColumn（name =User_Id，referencedColumnName =User_Id）
私人用户用户; 
 
 @ManyToOne 
 @PrimaryKeyJoinColumn（name =App_Id，referencedColumnName =App_Id）
私人App应用; 
 
 // Getters& SetApp方法
} 
  

在UserAppBridgeId类中：



  public class UserAppBridgeId {
 
 private long userAppBridgeId; 
 private int userId; 
 private int appId; 
 
 // Getters& Setter方法
} 
  

有关如何创建高级manyToMany关系的更多信息，请参阅< wiki链接






2. 它应该是一个SQL名称




3. 使用unique = true ，没有unique = true，它会是manyToMany。
   





说明：
让我们有以下一段代码：



  @OneToMany 
 @JoinTable {
 //这里应该是表名和而不是实体名称
 name =User_App_Bridge，
 joinColumns = {@JoinColumn（name =User_Id，referencedColumnName =User_Id）}，
 inverseJoinColumns = {@JoinColumn（name = App_Id，referencedColumnName =App_Id，unique = true）} 
} 
 private List< App>应用; 
  

如果存在unique = true属性，则映射表可以只有唯一的App_id相同的App_id在映射表中不会多次出现一次），而它可以具有任何User_id（相同的User_id可以在映射表中出现多次）。这意味着，一个User_id可能已经分配了更多的App_id，而一个App_id可能只分配了一个User_id。因此你强制一对多关系。





4. 是的，您确实如此。如答案1所示，您必须为UserAppBridge实体创建一个Id类，并为用户和应用程序实体添加一个映射






Am new to JPA...

I have the following preexisting tables inside a MySQL 5 database:

• App
• User
• User_App_Bridge

The relationship(s) are mapped to the User_App_Bridge table (as the name implies, a bridge) amongst App and User as a One to Many cardinality.

The E-R diagram:

User ------ User_App_Bridge------ App

1 User is associated with many apps inside the User_App_Bridge table.

1 App is associated with many users inside the User_App_Bridge table.

The DDL for the User_App_Bridge table:

CREATE TABLE `User_App_Bridge` (
    `User_App_Bridge_Id` int(11) NOT NULL AUTO_INCREMENT,
    `User_Id` int(11) NOT NULL,
    `App_Id` int(11) NOT NULL,
    PRIMARY KEY (`User_App_Bridge_Id`),
    KEY `App_Id` (`App_Id`),
    KEY `User_Id` (`User_Id`),
    CONSTRAINT `user_app_bridge_ibfk_1` FOREIGN KEY (`App_Id`) REFERENCES `App` (`App_Id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Lets say that I have the following JPA Annotated Classes which are mapped to these tables:

@Entity
public class App {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "App_Id")
    private long appId;

    @OneToMany
    @JoinTable
    (
        name = "UserAppBridge",
        joinColumns = { @JoinColumn(name="App_Id", referencedColumnName = "App_Id") },
        inverseJoinColumns = { @JoinColumn(name="User_Id", referencedColumnName = "User_Id") }
    )
    private List<User> users;

    @Column(name = "App_Name")
    private String appName;

    // Getters & Setter methods
}

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "User_Id")
    private long userId;

    @OneToMany
    @JoinTable
    (   
       name = "UserAppBridge",
       joinColumns = { @JoinColumn(name="User_Id", referencedColumnName = "User_Id") },
       inverseJoinColumns = { @JoinColumn(name="App_Id", referencedColumnName = "App_Id", unique = true) }
    )
    private List<App> apps;

    @Column(name = "User_Name")
    private String userName;

    // Getters & Setter methods
}

@Entity
public class UserAppBridge {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "User_App_Bridge_Id")
    private long userAppBridgeId;

    @Column(name = "User_Id")
    private long userId;

    @Column(name = "App_Id")
    private long appId;

    // Getters & Setter methods
}

Question(s):

1. Is this (the block inside the @JoinTable) the correct way to do the one to many mapping for User and App entites with the UserAppBridge?

2. Inside the @JoinTable should the column & referencedColumnName be assigned to the SQL value (e.g. name="User_Id" , referencedColumnName = "User_Id") or should it be the Java reference name (e.g. name="userId" , referencedColumnName = "userId")?

3. Inside the inverseJoinColumns code black, is the unique = true necessary (what is it for)?

4. Do I need to do anything else inside the UserAppBridge class (for connecting to App and User)?

Thank you for taking the time to read this...

解决方案

The answers for your questions:

1.No, it is not. You need to create a oneToMany mapping between User and UserAppBridge, and oneToMany mapping between App and UserAppBridge. Here is the code:

In User entity:

    @OneToMany(mappedBy = "user")
    private Set<UserAppBridge> userAppBridgeSet;

In App entity:

    @OneToMany(mappedBy = "app")
    private Set<UserAppBridge> userAppBridgeSet;

In UserAppBridge entity:

    @Entity
    @IdClass(UserAppBridgeId.class)
    public class UserAppBridge{

        @Id
        @Column(name = "User_App_Bridge_Id")             
        private long userAppBridgeId;

        @Id
        @Column(name = "User_Id")
        private long userId;

        @Id
        @Column(name = "App_Id")
        private long appId;

        @ManyToOne
        @PrimaryKeyJoinColumn(name="User_Id", referencedColumnName="User_Id")
        private User user;

        @ManyToOne  
        @PrimaryKeyJoinColumn(name="App_Id", referencedColumnName="App_Id")  
        private App app;

        // Getters & Setter methods
    }

In UserAppBridgeId class:

    public class UserAppBridgeId{

        private long userAppBridgeId;
        private int userId;
        private int appId;  

        // Getters & Setter methods 
    }

For more information on how to create advanced manyToMany relationship please see a wiki link

2. It should be an SQL names

3. With "unique=true" you are forcing a oneToMany relationship, without a "unique=true" it would be manyToMany.

Explanation: Lets have a following piece of code:

    @OneToMany
    @JoinTable{
        //Here should be the table name and not the entity name
        name = "User_App_Bridge",
        joinColumns = { @JoinColumn(name="User_Id", referencedColumnName = "User_Id") }, 
        inverseJoinColumns = { @JoinColumn(name="App_Id", referencedColumnName = "App_Id", unique = true) }
    }
    private List<App> apps;

if there is a "unique=true" attribute, your mapping table can have only unique App_id (the same App_id can not occure more then once in mapping table), while it can have any User_id (same User_id can occur more then once in mapping table). What this means, is that one User_id may have assigned more App_id, while a single App_id may have assigned only one User_id. Hence you force a oneToMany relationship.

4. Yes, you do. As shown in answer 1 , you will have to create an Id class for UserAppBridge entity and also add a mapping for User and App entities

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