title: ebean unidirectional @OneToOne relation with unique constraint
body: <p>I have a User class:</p>
  
  <pre><code>@Entity
  public class User extends Model {
  
  @Id
  public Long id;
  public String email;
  public String name;
  public String password;
  }
  </code></pre>
  
  <p>and a driver class</p>
  
  <pre><code>@Entity
  public class Driver extends Model {
  @Id
  public Long id;
  
  @OneToOne (cascade = CascadeType.ALL)
  @Column(unique = true)
  public User user;
  }
  </code></pre>
  
  <p>I want to make sure that the user_id is unique inside the Drivers table. But the code above does not enforce that. (I can create multiple drivers with the same user id).</p>
  
  <p>Ideally, I do not want to add the @OneToOne relations in the User class because there are several different roles inside my app (e.g. driver, teacher, agent etc.) and I don't want to pollute user class with all those relations.</p>
  
  <p>How can I achieve this?</p>