How can I retrieve all the children of a record in this Hibernate @ManyToOne relation?

I am working on a Spring Boot project using Spring Data JPA and Hibernate mapping. I have the following doubt about how can I implement the following query.

I have an User entity class like this:

@Entity
@Table(name = "portal_user")
@Getter
@Setter
public class User implements Serializable {
     
    private static final long serialVersionUID = 5062673109048808267L;
    
    @Id
    @Column(name = "id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;
    
    @Column(name = "first_name")
    @NotNull(message = "{NotNull.User.firstName.Validation}")
    private String firstName;
    
    @Column(name = "middle_name")
    private String middleName;
    
    @Column(name = "surname")
    @NotNull(message = "{NotNull.User.surname.Validation}")
    private String surname;
    
    @Column(name = "sex")
    @NotNull(message = "{NotNull.User.sex.Validation}")
    private char sex;
    
    @Column(name = "birthdate")
    @NotNull(message = "{NotNull.User.birthdate.Validation}")
    private Date birthdate;
    
    @Column(name = "tax_code")
    @NotNull(message = "{NotNull.User.taxCode.Validation}")
    private String taxCode;
    
    @Column(name = "e_mail")
    @NotNull(message = "{NotNull.User.email.Validation}")
    private String email;
    
    @Column(name = "pswd")
    @NotNull(message = "{NotNull.User.pswd.Validation}")
    private String pswd;
    
    @Column(name = "contact_number")
    @NotNull(message = "{NotNull.User.contactNumber.Validation}")
    private String contactNumber;
    
    @Temporal(TemporalType.DATE)
    @Column(name = "created_at")
    private Date createdAt;
    
    @Column(name = "is_active")
    private boolean is_active;
    
    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy = "user", orphanRemoval = true)
    @JsonManagedReference
    private Set<Address> addressesList = new HashSet<>();
    
    @ManyToMany(cascade = { CascadeType.MERGE })
    @JoinTable(
        name = "portal_user_user_type", 
        joinColumns = { @JoinColumn(name = "portal_user_id_fk") }, 
        inverseJoinColumns = { @JoinColumn(name = "user_type_id_fk") }
    )
    Set<UserType> userTypes;


    @ManyToOne(fetch = FetchType.EAGER)
    @JsonProperty("subagent")
    private User parent;

    public User() {
        super();
    }


    public User(String firstName, String middleName, String surname, char sex, Date birthdate, String taxCode,
            String email, String pswd, String contactNumber, Date createdAt, boolean is_active) {
        super();
        this.firstName = firstName;
        this.middleName = middleName;
        this.surname = surname;
        this.sex = sex;
        this.birthdate = birthdate;
        this.taxCode = taxCode;
        this.email = email;
        this.pswd = pswd;
        this.contactNumber = contactNumber;
        this.createdAt = createdAt;
        this.is_active = is_active;
    }
    

}

The instances of this class represents users of my system. An user can have a single specific parent (the concept is similar to that of a referral: an user can bring another user in the system). This is handled by this ManyToOne recursive relationship:

@ManyToOne(fetch = FetchType.EAGER)
@JsonProperty("subagent")
private User parent;

Basically an user contains is parent (who bring him\her into the platform). It works fine. So retrieving an user I can easily retrieve the information of who is its parent (it is contained into the retrieved User object).

Now I need to implement the inverse behavior: I have to define a "query" that starting from a parent retrieve all its children.

The previous User entity class maps the following DB table:

The highlighter parent_id contains the FK that define this recursive relationship. So it contains the PK of another user that is the parent.

I have this UserRepository repository interface (it extents the JpaRepository interface)

public interface UsersRepository extends JpaRepository<User, Integer> {
    
    User findByemail(String email);
    
    List<User> findByUserTypes_TypeName(String typeName);

}

As you can see I am using a "query by method" style. Is it possiblem implement a behavior like this using "query by method" style? (in case also JPQL could be fine)

>Solution :

You can do this

List<User> findByParent_Id(Integer id);

Or you can do this

@Query("SELECT u FROM User u WHERE u.id = ?1")
List<User> getReferredUsers(Integer id);