title: Hibernate nullable=true not working in @ManyToOne relationship
body: <p>This is my scenario,
  I have Cars, which belong either to a Dealer or a User (not both at the same time).</p>
  
  <p>So my Car entity has a @ManyToOne relation with Dealer and User, but one of them needs to be null, so I added nullable=true in both, to support that.</p>
  
  <p>This is my code:</p>
  
  <pre><code>@Entity
  @Table(name = "car")
  @XmlRootElement
  public class Car  implements Serializable{
  
      @Id
      @Column(name = "id", unique = true, nullable = false, columnDefinition = "serial")
      @SequenceGenerator(name = "pk_sequence", sequenceName = "car_id_seq", allocationSize = 1)
      @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "pk_sequence")
      @Generated(GenerationTime.INSERT)
      private Long id;
  
      @ManyToOne
      @JoinColumn(name = "dealer", nullable = true)
      private Dealer dealer;
  
      @ManyToOne
      @JoinColumn(name = "user", nullable = true)
      private User user;
  </code></pre>
  
  <p>But then when I try to insert a car without a Dealer I get this:</p>
  
  <p>ERROR: null value in column "dealer" violates not-null constraint.</p>
  
  <p>I understand that is because of a foreign key restriction generated by Hibernate in the database. But then:
   -What is the reason of having "nullable = true"?
   -How can I solve this problem where I need to have a @ManyToOne relationship and it can be null?</p>
  
  <p>Thanks.</p>