title: node program with promises doesn't finish
body: <p>I cannot for the life of me figure out why this doesn't finish:</p>
  
  <pre><code>var pmongo = require('promised-mongo');
  var db = pmongo('mongodb://localhost/builder');
  var block_id = '538d097bbb12479d0e9f70ab';
  
  var collection = db.collection('block');
  collection.findOne({_id:db.ObjectId(block_id)})
  .then(function(result) {
      console.dir(result);
  }).done();
  </code></pre>
  
  <p>It bascially just hangs.  findOne returns a promise, I'm calling done.  Strangely, when I close the database (ie db.close()) in the then, it finishes. </p>
  
  <p>I'm trying to eventually make this handled via express, so I don't really want to close the database.  What's the trick????</p>