title: How can I sort into that nulls are last ordered in mongodb?
body: <p>I excute this query in the mongo shell </p>
  
  <pre><code>db.getCollection('list').find({}).sort({next_time: 1})
  </code></pre>
  
  <p>the result is </p>
  
  <pre><code>next_time
  --
  null
  null
  null
  2015-02-21 00:00:00
  2015-03-25 00:00:00
  2015-08-29 00:00:00
  </code></pre>
  
  <p>I hope to make the result like this</p>
  
  <pre><code>next_time
  --
  2015-02-21 01:00:00
  2015-03-25 01:00:00
  2015-08-29 01:00:00
  null
  null
  null
  </code></pre>
  
  <p>The different is that 'null's are last ordered in the list. How can I do for this?</p>