title: warn user if all checkboxes are unchecked
body: <p>I have a form with a series of checkboxes.  I want to warn the user, after they hit submit, if ALL of the checkboxes are unchecked.  I'm using the following code to report all of the values of the checkboxes:</p>
  
  <pre><code>$('[id^=leg_rider]').filter(':checked');
  </code></pre>
  
  <p>This seems to work.  However, when I try to check to see if the returned object is empty, it doesn't seem to work.  This is what I'm trying:</p>
  
  <pre><code>        $("#set_pref_submit").click(function() {
          var legchecked = $('[id^=leg_rider]').filter(':checked');
          if (!legchecked){ alert("no riders")};
      });
  </code></pre>
  
  <p>Any suggestions are appreciated.  Thanks!</p>