从 spring jpa 中的多到多列中检索数据

Question

I have user entity and category entity and they are as followed.One user can subscribe to many categories and similarly same category can be subscribed by many users.

@Data
@AllArgsConstructor
@NoArgsConstructor
@Entity
@Table(name="users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long userId;

   @NotBlank(message = "username can't be blank")
    private String username;

    @Email
    @NotEmpty(message = "Email is required")
    @Column(unique=true)
    private String email;

    @NotBlank(message = "Password is required")
    private String password;

  
    // Each user is going to be mapped to a Location
    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(
            name = "location_id",
            referencedColumnName = "locationId"
    )
    @NotNull
    private Location location ;


    @EqualsAndHashCode.Exclude
    @ToString.Exclude
    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
    @JoinTable(name = "users_categories",
            joinColumns = {
                    @JoinColumn(name = "user_id", referencedColumnName = "userId",
                            nullable = false, updatable = false)},
            inverseJoinColumns = {
                    @JoinColumn(name = "category_id", referencedColumnName = "categoryId",
                            nullable = false, updatable = false)})
    private Set <Categories> categories = new HashSet<>();


    
}

And categories entity

@Data
@AllArgsConstructor
@Entity
@Table(name="categories")
public class Categories {

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


    @NotBlank(message = "Please Add Category name ")
    @Length(max =100 ,min =2)
    private String categoryName ;


    @Length(max=500,min=2)
    private String categoryDescription;

    @EqualsAndHashCode.Exclude
    @ToString.Exclude
    @ManyToMany(mappedBy = "categories", fetch = FetchType.EAGER)
    private Set<User> users = new HashSet<>();

    
}

and I have this table where user and categories are mapped.

mysql> select * from users_categories where category_id=5;
+---------+-------------+
| user_id | category_id |
+---------+-------------+
|       3 |           5 |
|       7 |           5 |
+---------+-------------+
2 rows in set (0.00 sec)

I want to retrieve all userId with a specific category. How do we achieve this in spring boot and JPA? Here is my user repository

import java.util.Optional;
@Repository
public interface UserRepository extends JpaRepository<User, Long>{
    Optional<User> findByUsername(String username);
}

Answer 1

I didn't test it but try to add this to your UserRepository :

  @Query("select u from User u inner join u.categories c where c.categoryId =:id")
  List<User> findByCategory(@Param("id") Long id);