Many to many relationship for same type entity

Question

I have an entity as below. I am curious if it is possible to create a relationship as I will be describing with the example:

• I am creating 2 Person entities Michael and Julia.
• I am adding Julia to Michael's friends set.

• After that I am retrieving Michael as a JSON response and Julia is available in the response. But when I am retrieving Julia, her friends set is empty. I want to create the bidirectional friendship relation by saving just one side of the friendship. I would like to get Michael on Julia's friends set without doing any other operations. I think that it must be managed by Hibernate. Is it possible and how should I do it?

  @ToString(exclude = "friends") // EDIT: these 2 exclusion necessary
  @EqualsAndHashCode(exclude = "friends")
  public class Person{
  
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  @Column(name = "id")
  private Long id;
  
  @Column(name = "name",unique = true)
  private String name;
  
  @JsonIgnoreProperties("friends") // EDIT: will prevent the infinite recursion
  @ManyToMany(cascade = CascadeType.ALL)
  @JoinTable(name = "FRIENDSHIP",
         joinColumns = @JoinColumn(name = "person_id", 
  referencedColumnName = "id"),
         inverseJoinColumns = @JoinColumn(name = "friend_id", 
  referencedColumnName = "id"))
  private Set<Person> friends;
  

Here is my service layer code for creating a friendship:

    @Override
    public Person addFriend(String personName, String friendName) 
        throws FriendshipExistsException, PersonNotFoundException {
    Person person = retrieveWithName(personName);
    Person friend = retrieveWithName(friendName);
    if(!person.getFriends().contains(friend)){
        person.getFriends().add(friend);
        return repository.save(person);
    }
    else{
        throw new FriendshipExistsException(personName, friendName);
    }
}

Related Question: N+1 query on bidirectional many to many for same entity type

Updated the source code and this version is working properly.

Answer 1

// Creating a graph to help hibernate to create a query with outer join.
@NamedEntityGraph(name="graph.Person.friends",
    attributeNodes = @NamedAttributeNode(value = "friends"))
class Person {}

interface PersonRepository extends JpaRepository<Person, Long> {
    // using the named graph, it will fetch all friends in same query
    @Override
    @EntityGraph(value="graph.Person.friends")
    Person findOne(Long id);
}

@Override
public Person addFriend(String personName, String friendName) 
    throws FriendshipExistsException, PersonNotFoundException {
    Person person = retrieveWithName(personName);
    Person friend = retrieveWithName(friendName);
    if(!person.getFriends().contains(friend)){
        person.getFriends().add(friend);
        friend.getFriends().add(person); // need to setup the relation
        return repository.save(person); // only one save method is used, it saves friends with cascade
    } else {
        throw new FriendshipExistsException(personName, friendName);
    }

}

If you check your hibernate logs, you will see:
Hibernate: insert into person (name, id) values (?, ?)
Hibernate: insert into person (name, id) values (?, ?)
Hibernate: insert into friendship (person_id, friend_id) values (?, ?)
Hibernate: insert into friendship (person_id, friend_id) values (?, ?)