title: MongoDB projection parameter not working in findOne()
body: <p>I'm trying to use a projection parameter on findOne() to extract a single field from a document (stats) but it just seems to return the whole document. I'm using version "mongodb": "^3.4.1" in Node.js</p>
  
  <p>This is the document structure</p>
  
  <pre><code>{ _id: 5e563015fa9a1a0134cac3cb,
    username: 'user1',
    password: '1234',
    email: 'user@email.com',
    stats: 
     { totalViewed: 122,
       totalUnique: 4,
       tknow: 80,
       tdknow: 42,
       setCnt: 78 },
    progress: 
     [ { cardId: 1001, knowCnt: 3, dknowCnt: 4 },
       { cardId: 1016, knowCnt: 0, dknowCnt: 0 } ] }
  
  </code></pre>
  
  <p>This is the code:</p>
  
  <pre><code> var findOneDoc = function() {
          db.collection("testusers").findOne(
            { username: "user1" },
            { stats: 1 }, //field to return
            function(err, result) {
              if (err) {
                console.log("Error: ", err);
              }
              console.log("Success: ", result);
            }
          );
        };
       findOneDoc();
  </code></pre>
  
  <p>I also tried:<code>{$project: {stats: 1}}</code>, to no avail</p>
  
  <p>Thanks</p>