title: Synchronous findOne with sort() in mongoose mongdb
body: <p>I want to find document with the latest "sorting_index" field.
  I try so:</p>
  
  <pre><code>var sorting_index = -1;
  this.findOne().sort({'sorting_index': -1}).exec(function(err, doc) {   
      if (err) return;
      sorting_index = doc['sorting_index']; // sorting_index == 10      
  });                         
  console.log(sorting_index); // sorting_index == -1
  </code></pre>
  
  <p>The problem is that the callback is asynchronous. How to do it synchronically?</p>