title: constraint in Django: exactly 1 of 2 foreign keys has to be null
body: <p>I'd like to allow null on two foreign key fields.</p>
  
  <p>But the two fields should not be null at the same time.<br>
  Actually, exactly one has to be set at any time.  </p>
  
  <p>How can I express this? </p>
  
  <p>The two tables the foreign keys reference are not the same. </p>