Many to many relationship for same type entity

问题

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.

回答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 (?, ?)

来源：https://stackoverflow.com/questions/48551459/many-to-many-relationship-for-same-type-entity

标签

java

Spring

hibernate

many-to-many

bidirectional-relation