title: JPA ManyToMany: set/update by list of ids instead of list of actual objects
body: <p>I have two domain classes with a many-2-many relationship between them, e.g. User and Group.  There's a user2group table in the database to map this relationship. If I want to set User's Groups based on a list of Group IDs (which might come e.g. from a HTML form) do I really have to fetch the Groups first to update the User through <code>setUsers(Set&lt;Users&gt;)</code>? I already have the Group IDs and they are actually the only information needed to update the mapping table...</p>
  
  <p>I'm using Spring Data JPA and Hibernate as the provider.</p>