aggregate returns empty array - mongoose
<p>I have the following two simple queries:  </p>

<pre><code>Comment.aggregate([{$match: { _id: req.params.id }}])
  .exec(function(err, result) {
    // result is empty
  });

Comment.find({ _id: req.params.id })
  .exec(function (err, result) {
    // correct result returned
  });
</code></pre>

<p>My problem is, that the aggregate-Function returns an empty array. Aren't they supposed to return the same result? </p>