title: @MappedSuperclass is not an @Entity in JPA?
body: <p>So I am using DerbyDB and I am setting up some entities. I have a <code>@MappedSuperclass</code> which is used as a superclass for some entities (<code>@Entity</code>). More specifically, I have a superclass <code>User</code> and 3 subclasses namely <code>admin</code>, <code>regular</code> and <code>guest</code>. Now I have a different entity, let's say <code>file</code> that should reference (as one of its fields) its owner. So I created a field called <code>User owner</code>.
  The error I get is:</p>
  
  <pre><code>Exception Description: [File] uses a non-entity [User] as target entity in the relationship attribute [field owner].
  </code></pre>
  
  <p>Is there a workaround?</p>