title: JPA foreign key - id or reference
body: <p>I got two classes: <code>Forum</code> and <code>Topic</code>.<br>
  I previously told I should have Forum attribute (in Topic), something like that:  </p>
  
  <blockquote>
    <p>@ManyToOne<br>
      @JoinColumn(name = "forum")<br>
      protected Forum forum;</p>
  </blockquote>
  
  <p>instead of just keeping the id of the forum.<br>
  I saw that in postgresql the Forum attribute saved as "bigint" (the forum id)
  so what's the point of holding the Forum reference anyway?  </p>