Spring Boot JPA 如何从 ManyToMany 表中检索数据？答案

【问题标题】：Spring Boot JPA how to retrieve data from a ManyToMany table?Spring Boot JPA 如何从 ManyToMany 表中检索数据？
【发布时间】：2021-03-03 10:09:09
【问题描述】：

我正在学习 Spring Boot。现在我与一个用户实体和一个团队实体有多对多关系。所以一个用户可以有多个团队，一个团队可以有多个用户。

   @Entity
    public class User {
        @Id
        @GeneratedValue(strategy=GenerationType.IDENTITY)
        @Column(name="id", nullable =false, updatable = false)
        private Long id;
@ManyToMany
    @JoinTable(
            name = "team_have",
            joinColumns = @JoinColumn(name = "id"),
            inverseJoinColumns = @JoinColumn(name = "teamId"))
    List<Team> haveTeams;

这是团队实体：

@Entity
@Table(name="TEAM")
public class Team {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long teamId;
    @OneToMany(cascade = CascadeType.ALL, mappedBy="team")
    private List<Task> tasks;

这是我的仓库：

public interface TeamRepository extends CrudRepository<Team, Long>{
    List<Team>findByName(String name);
    
}

如何找到属于一个用户的所有团队？

【问题讨论】：

将此添加到您的团队存储库中：ListfindByUser(User user);

标签： java spring-boot jpa spring-data-jpa spring-data

【解决方案1】：

用户已经使用List<Team>进行映射，您需要使用UserRepository而不是TeamRepository。

public interface UserRepository extends CrudRepository<User, Long>{
    User findByName(String name);
    
}

在这里，您将获得一个用户，该用户拥有他所属的所有团队。（假设用户名是唯一的）

或

如果您有双向多对多（团队也映射Lis<User>），如下所示

@Entity
@Table(name="TEAM")
public class Team {

    ....
    @ManyToMany(cascade = CascadeType.ALL, mappedBy="haveTeams")
    private List<User> users;

然后您可以定义如下查询方法来获取一个用户的所有团队，

public interface TeamRepository extends CrudRepository<Team, Long>{
    List<Team> findByUsers(User user);
    
}

【讨论】：

【解决方案2】：

在它们之间建立连接的最佳方式是使用双向关系，如下所示：

User Entity

@Entity
@Table(name = "user")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    /* @Column(name = "id", nullable = false, updatable = false) You don't need this, 
    the name is taken from the name of the variable and it cannot be null since you 
    have @GeneratedValue */
    private Long id;

    @ManyToMany(fetch = LAZY, cascade = CascadeType.PERSIST)
    @JoinTable(
        name = "user_team", // I would name this user_team since the user can have many teams, and vice versa
        joinColumns = @JoinColumn(name = "user_id" , referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "team_id",  referencedColumnName = "id"))
     Set<Team> teams = new HashSet<>();
}

Team Entity

@Entity
@Table(name="team")
public class Team {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long teamId;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "team")
    private List<Task> tasks;

    @ManyToMany(mappedBy = "teams", fetch = LAZY, cascade = CascadeType.PERSIST)
    private Set<User> users = new HashSet<>();
}

Team Repo

public interface TeamRepository extends CrudRepository<Team, Long>{
     @Query(value = "SELECT * FROM team t1 INNER JOIN user_team t2 ON t1.id = t2.team_id WHERE t2.user_id = ?1 ")
    List<Team> findAllByUserId(long userId);
}

使用示例：

List<Team> teams = teamRepo.findAllByUserId(1);

一个很棒的教程： https://attacomsian.com/blog/spring-data-jpa-many-to-many-mapping

【讨论】：