Question

Lets say we have User entity class. User can be friends with other users. How can i map this self-reference collection field without creating a new entity called Connection or creating multiple entries in the database?

@Entity
public class User {
...
@ManyToMany
private Collection<User> friends;
...
}

USER_ID-FRIEND_ID
1 - 2
2 - 1 (duplicate... I don't need it)

Answer 1

You can’t – you need both records in the database.

Actually, for friendship relations, I’d say that a graph database like neo4j is the proper thing to use. There you have the two users and simply add an edge “friends”.