title: Hibernate validation annotation - validate that at least one field is not null
body: <p>Is there a way to define a Hibernate validation rule using annotations as defined <a href="http://docs.jboss.org/hibernate/validator/4.0.1/reference/en/html_single/#validator-customconstraints-compound" rel="noreferrer">here</a>, stating that at least one field shall be not null?</p>
  
  <p>This would be a hypothetical example (<code>@OneFieldMustBeNotNullConstraint</code> does not really exist):</p>
  
  <pre><code>@Entity
  @OneFieldMustBeNotNullConstraint(list={fieldA,fieldB})
  public class Card {
  
      @Id
      @GeneratedValue
      private Integer card_id;
  
      @Column(nullable = true)
      private Long fieldA;
  
      @Column(nullable = true)
      private Long fieldB;
  
  }
  </code></pre>
  
  <p>In the illustrated case, fieldA can be null or fieldB can be null, but not both.</p>
  
  <p>One way would be to create my own validator, but I'd like to avoid if it already exists. Please share one validator if you have one already made... thanks!</p>