title: Mongoid OR query syntax
body: <p>This must be asked alot but it is very poorly documented. There is no mention at <a href="http://mongoid.org/en/mongoid/docs/querying.html" rel="noreferrer">http://mongoid.org/en/mongoid/docs/querying.html</a></p>
  
  <p>I'm trying to check whether a user exists (below is an AND query), how can I change it to an <code>OR</code> type query</p>
  
  <pre><code>Username.where(:username=&gt;@username, :email=&gt;@email)
  </code></pre>
  
  <p>(Either the email or the username must match).</p>
  
  <p>I have found some pretty complicated ways online including sending a direct javascript (from): <a href="http://omarqureshi.net/articles/2010-6-17-using-or-in-mongoid" rel="noreferrer">http://omarqureshi.net/articles/2010-6-17-using-or-in-mongoid</a></p>
  
  <p>Surely there must be a simple clear syntax to do this correctly?</p>