How can I retrieve all the children of a record in this Hibernate #ManyToOne relation? - spring-boot

Question

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)

Answer 1

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);

Answer 2

The relationship between the user and the parent is unidirectional in the given code. By making it bidirectional, it is easy to query the data in either ways.
Refer to below code to make it bidirectional. Also ensure the relevant FetchType to avoid the performance risk. Here FetchType.LAZY is used for one to many association so it queries the data using the proxy reference when needed.
#ManyToOne(fetch = FetchType.EAGER)
#JsonProperty("subagent")
#JsonBackReference
private User parent;
#JsonManagedReference
#OneToMany(fetch = FetchType.LAZY, mappedBy = "parent")
private Set<User> userSet = new HashSet<>();
Child entities are fetched only when parent.getUserSet is used because of the FetchType.Lazy
public Set<User> getUsers(int id) {
User parent = userRepository.getById(id);
return parent.getUserSet();
}