title: How to do a validation conditional in this case with Laravel?
body: <p>I have two fields. <code>author_id</code> and <code>teacher_id</code>. I would like one or the other to be required BUT if both are filled, it displays an error saying "there can be only one".</p>
  
  <p>Is it possible to do this with validation rules in Laravel? I do not understand the documentation. Perhaps an example will be more meaningful (and an explanation)</p>