title: JPA: foreign key annotation
body: <p>I've got two database entities: <code>Forum</code> and <code>Topic</code>.  </p>
  
  <p><code>Topic</code> has <code>protected long forumId</code> data member, which indicates of course the <code>Topic</code>'s forum.  </p>
  
  <p>My question is what annotation to use for this data member?</p>