I have a `users(id)` table and a `friends(user_id1, user_id2)` where `user_id1` and `user_id2` are both foreign keys to the `users` table.

What relationship is this ?

I think is One to Many, but since I have the user id two times it's a bit confusing. 
(I have to make a diagram and don't know what relationship to use).