如何在双向关系中删除一侧的对象？

Question

我想从一边删除关系。 让我解释。

我的数据库看起来像这样：

我将我的存储库建立在JPARepository 。

我有两个实体Team和Player 。

Player实体看起来像这样：

@Entity
    @Table(name = "PLAYER")
    public class Player implements Serializable, Comparable<Player>{
        private static final long serialVersionUID = 1L;

        @Id
        @GeneratedValue(strategy=GenerationType.AUTO)
        private Long id_player;

        @Size(min=3, max=20)
        private String name;
        @Size(min=3, max=20)
        private String surname;
        @Size(min=3, max=20)
        private String position;
        @Min(value=10) 
        private int age;
        @Min(value=1)
        private int number;

        @ManyToOne(fetch = FetchType.LAZY)
        @JoinColumn(name = "id_team")
        private Team team;
        //getters and setters
    }

......和Team实体：

@Entity
@Table(name = "TEAM")
public class Team implements Serializable, Comparable<Team>{
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id_team;
    @Size(min=4, max=20)
    private String name;
    @Size(min=4, max=20)
    private String city;
    private int goals_hit;
    private int goals_lost;
    private int points;
    private int ranked;

    @OneToOne(cascade = CascadeType.MERGE)
    @JoinColumn(name = "id_coach")
    private Coach coach;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "id_stadium")
    private Stadium stadium;

    @OneToMany(mappedBy = "team", fetch = FetchType.LAZY)
    private List<Player> playerList;

    @OneToMany(mappedBy = "team", fetch = FetchType.LAZY)
    private List<TeamChoosenDate> teamChoosenDateList;
    //getters and setters
}

因此， Player包含与Team一个关系，但Team包含一个Players List 。

教练可以编辑他的团队，例如他想要从Team删除Player （ Team - > Player之间的关系），但不要失去Player （因为他与其他表有关系 - Events ）。

总之：从正确的Team删除Player ，但不要丢失Player对象。

我怎样才能做到这一点 ？

我的尝试：我从List删除Player ，但每次重新加载时 - 他都回来了......下面的方法显示了我是如何做到的：

public void deletePlayerFromTeamPlayerList(Long idTeam, int idPlayer){

    Team team = teamRepository.findOne(idTeam);
    Player playerTmp = playerRepository.findOne( (long) idPlayer);

    List<Player> playerList;

    int indexOfFindedPLayer;


    playerList = team.getPlayerList();

    System.out.println("Before Delete: " + playerList.toString());

    indexOfFindedPLayer = playerList.indexOf(playerTmp);

    if(indexOfFindedPLayer >= 0){
        playerList.remove(indexOfFindedPLayer);

        System.out.println("After Delete: " + playerList.toString());

        team.setPlayerList(playerList);
        teamRepository.save(team);
    }
}

但是当元素被删除时，在Player对象中id_team仍然是相同的（例如id_team = 2）

谢谢你的所有建议！

Answer 1

您可以通过以下方式更简单地实现此目的：

public void deletePlayerFromTeam(int idPlayer){
    Player player = //loadPlayer;
    player.setTeam(null);

    playerRepository.save(player);
}

您也可以通过将player.team设置为null 并在集合上设置cascade来合并，从而在原始代码中实现此目的：

@OneToMany(mappedBy = "team", fetch = FetchType.LAZY, cascade = CascadeType.MERGE)
private List<Player> playerList;

您应该始终确保关系的两个方面保持一致。

Answer 2

使用此方法仅删除一对多或多对一关系中的一侧。

@ManyToOne(cascade=CascadeType.PERSIST, fetch = FetchType.LAZY)
//  @JoinColumn(name = "qid")
    @JoinColumn(name = "qid", referencedColumnName = "qid", foreignKey = @ForeignKey(name = "qid"), nullable = false)
    // @JsonIgnore
    @JsonBackReference
    private QueueGroup queueGroup;

这里CascadeType是所有事务所依赖的主要内容。