title: MongoError unknown group operator
body: <p>I am using MongoDb Driver for NodeJS.</p>
  
  <p>I am facing issues in using aggregation. 
  The error is</p>
  
  <blockquote>
    <p>{"name":"MongoError","message":"unknown group operator
    '_id'","ok":0,"errmsg":"unknown group operator '_id'","code":15952}</p>
  </blockquote>
  
  <p>for the below script : </p>
  
  <pre><code>MongoClient.connect(url, function (err, db) {
        if (err) 
        {
          console.log('Unable to connect to the mongoDB server. Error:', err);
          return;
        }
      var collName = "order";
      var whereParas = {};
      var groupParas = {"_id":null,total:{$sum:"$Value"}};
      var havingParas = {};
  
              db.collection(collName).aggregate(
              [
              { $match: whereParas },
              {
                $group: { groupParas}
              },
              { $match: havingParas }
          ]).toArray(function (err,result) {
              console.log("err");
              console.log(err);
              console.log("result");
              console.log(result);
  
              });
  });
  </code></pre>
  
  <p>The data used is <a href="https://i.stack.imgur.com/aKqvb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aKqvb.png" alt="enter image description here"></a></p>
  
  <p>Desired Output is Sum of Values.</p>
  
  <p>In sql, I would have written :</p>
  
  <pre><code>Select Sum(Value) From order
  </code></pre>