title: Get only change node value in firebase
body: <p>I am using the <code>ref.on()</code> function to monitor the change in firebase nodes, 
  but there is a problem, <code>ref.on()</code> gives whole database record.
  How can I get only those rows/nodes whose state is changed</p>
  
  <pre><code>function watch()
  {
      riderequests.once('value', function(snapshot)
         {
          alert("change");
          var newrequest = snapshot.val();
        }
    }
  </code></pre>