Question

I’m using:

• Hibernate
• MySQL
• jBoss

I have to create an application that allows user to make friendship.

I’m still in design phase, and I would like to have these tables:

- User {_id, name, address} 
 - Friendship {idUserA, idUserB, date}

For my purpose I should have a mutual relationship from Users. For example 1 is friend of 2 and indirectly 2 is friend of 1.
So in my table I think to have:

Friendship = {1, 2, …}

I’m asking how I can simulate this behaviour. I’ve read his topic: Many-to-many on the same table with additional columns and I’ve followed this instruction generating database tables from entity (property of hibernate). But in this example when I load my Friend A, I’ve a list with Friend B and when I load B I haven’t user A into List.

How I can implement this damn mutual relationship?

Answer 1

As far as I understand this sample the mutuality is not implicit. To have friendship relation from A->B and from B->A you have to either add two entries into your table or you have to implement two sets and return an intersection of the two list:

@OneToMany(mappedBy="myFriends")
private List<MyFriends> myFriends;

@OneToMany(mappedBy="me")
private List<MyFriends> iAmFriendOf;

...

Set<MyFriends> getFriends() {
    Set<MyFriends> friends = new HashSet<MyFriends>();
    friends.addAll(myFriends);
    friends.addAll(iAmFriendOf);
    return friends;
}