title: Symfony 2 onDelete setNull
body: <p>please can someone tell me how can I set onDelete: SetNull in Symfony 2? 
  Here is example:</p>
  
  <pre><code>oneToOne:
      Company:
          targetEntity: Company
          inversedBy: id
          joinColumn:
              name: id
              referencedColumnName: contact_person
              onDelete:  ??
  </code></pre>
  
  <p>Thanks.</p>