Thanks to this post, a clear explaination about how to specify a Many-to-many relation between 2 entities with a <master> and <slaves>.
When you map a same "ManyToMany relationShip" in both directions, youFor example : if you have two entities : "Event" and "User", and a
need to tell which of these two directions is (kind of) the "master".
So : one of your @ManyToMany annotations must have the info :
User can "follow" several "Events",
and an Event can be followed by several Users ("ManyToMany
relationShip"). You would have : in the entity User :
public Set<Event> followedEvents = new HashSet<Event>(); in the entity Event :
public Set<User> followsByUsers = new HashSet<User>();
Thanks to Dam74 !