持久性单元无法使关系多对多

Question

好吧，这是一个奇怪的问题，因为我以前有过它的工作。所以我有2个类：游戏和开发人员。他们有多对多的关系。持久性单元使表自动game_developer。因此有 3 个表：游戏开发人员和game_developer。当我在数据库中输入信息时。桌面游戏和开发人员将正常获得值，但表game_developer将保持为空。所以这种关系不被认可。此外，当我运行 web 应用程序时，应该删除表中的所有内容。开发商很好，但游戏仍然存在。

任何朝着正确方向的推动都是值得赞赏的。

提前谢谢你，

大卫

初始化：

try {
        gameOrganizer = (GameOrganizer) getServletContext().getAttribute("database");
        Game game = new Game("Counter Strike: source");
        Game game2 = new Game("Battlefield: bad company 3");
        Game game3 = new Game("Killing floor");
        Developer devel = new Developer("valve");
        Developer devel2 = new Developer("EA Games");
        Developer devel2b = new Developer("DICE");
        Developer devel3 = new Developer("Ubisoft");
        //The GameOrganizer is the controller between the model en the view. 
        //The view being the website.
        //So gameOrganizer.addGame(game) will persist the object to the database.
        gameOrganizer.addGame(game);
        gameOrganizer.addGame(game2);
        gameOrganizer.addGame(game3);
        gameOrganizer.addDeveloper(devel);
        gameOrganizer.addDeveloper(devel2);
        gameOrganizer.addDeveloper(devel2b);
        gameOrganizer.addDeveloper(devel3);
        game.addDeveloper(devel);
        devel.getGames().add(game);
        game2.addDeveloper(devel2);
        devel2.getGames().add(game2);
        game2.addDeveloper(devel2b);
        devel2b.getGames().add(game2);
        game3.addDeveloper(devel3);
        devel3.getGames().add(game3);
    } catch (DatabaseException ex) {
        Logger.getLogger(GameController.class.getName()).log(Level.SEVERE, null, ex);
    } catch (DomainException ex) {
        Logger.getLogger(GameController.class.getName()).log(Level.SEVERE, null, ex);
    }

游戏类：

@Entity
public class Game implements Serializable{
    private String gameNaam;
    private double prijs;
    @ManyToMany(mappedBy = "games")
    private Collection<Developer> developers = new ArrayList<Developer>();
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    public Game(){};
    .
    .
    //methods
    .
    .
  }

开发人员类

@Entity
public class Developer implements Serializable  {
    private String naam;
    private String info;
    @ManyToMany
    private Collection<Game> games = new ArrayList<Game>();
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    public Developer() {
    }
    .
    .
    //methods
    .
    .
 }

持久性单元：

  <?xml version="1.0" encoding="UTF-8"?>
 <persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
 <persistence-unit name="GameDatabaseSitePU" transaction-type="RESOURCE_LOCAL">
   <provider>oracle.toplink.essentials.PersistenceProvider</provider>
    <class>domainmodel.Developer</class>
    <class>domainmodel.Game</class>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
    <properties>
      <property name="toplink.jdbc.user" value="app"/>
      <property name="toplink.jdbc.password" value="app"/>
      <property name="toplink.jdbc.url" value="jdbc:derby://localhost:1527/GameDatabase;create=true"/>
      <property name="toplink.jdbc.driver" value="org.apache.derby.jdbc.ClientDriver"/>
      <property name="toplink.ddl-generation" value="drop-and-create-tables"/>
    </properties>
  </persistence-unit>
</persistence>

Answer 1

无法识别关系，因为在建立关系之前保留了对象。在将对象添加到彼此集合后gameOrganizer.add..()执行调用。此外，将级联持久性添加到开发人员的多对多关系中：

@ManyToMany(cascade = CascadeType.PERSIST, mappedBy = "games")
private Collection<Developer> developers = new ArrayList<Developer>();

然后，您只需要保留游戏。

Answer 2

我认为在双向多对多关系中，您需要设置双方......

即如果你这样做

devel.getGames（）.add（game）;

你还需要做

game.getDeveloper（）.add（devel）;

否则 JPA 不会真正看到另一边，插入到映射表中的内容将是类似（FK1，null）的内容，这将失败，这就是为什么您不会看到任何条目的原因......

我对toplink了解不多，但我认为类似

<property name="toplink.logging.level" value="FINE"/>

应该在日志记录中为您提供SQL语句，这有助于调试此类问题:)