title: Mongoose Error: Arguments must be aggregate pipeline operators
body: <p>I cannot get a result with aggregation option parameter. Here is my aggregation:-   </p>
  
  <pre><code>var emails = getAllEmails();
  var listMatchColl = 'list_matches_' + insertedId;
  SurveyDL.aggregate([
       { $match: { email: { $in: emails } } },
       { $out: listMatchColl }
   ], {
     allowDiskUse: true
   }).exec(function(err, data) {
    if (err) return console.log('err', err);
      console.log('data',data);
   });
  }
  </code></pre>
  
  <p>When i execute above code it thrown and error i.e,</p>
  
  <blockquote>
    <p>Error: Arguments must be aggregate pipeline operators
        at Aggregate.append (/home/vivek/nrich/node_modules/mongoose/lib/aggregate.js:89:11)
        at new Aggregate (/home/vivek/nrich/node_modules/mongoose/lib/aggregate.js:48:17)</p>
  </blockquote>
  
  <p>I have also use the alternative way but it still throw same error. Alternative way :-</p>
  
  <pre><code>var emails = getAllEmails();
  var listMatchColl = 'list_matches_' + insertedId;
  SurveyDL.aggregate([
       { $match: { email: { $in: emails } } },
       { $out: listMatchColl }
   ], {
     allowDiskUse: true
   },function(err, data) {
    if (err) return console.log('err', err);
      console.log('data',data);
   });
  </code></pre>