title: FindOneAndUpdate not working correctly for me
body: <p>I have an application where I have a database of instructors with an array of ID's for classes.  When a new class is created, I'm trying to find the instructor's database entry and update the classID's array with the new class's ID. However, when I try and update it, nothing happens.</p>
  
  <p>This is my code:</p>
  
  <pre><code> data.classID = id;
  new course(data).save((error) =&gt; {
      if(error){
          console.log('oops! Could not save course');
      } else {
          conn3.close();
      }
  });
  //update instructor's classID's variable in intructor database
  instructor.findOne({"instructorEmail":data.Instructor}, (err,x)=&gt;{
          var arr = x.classIDs;
          arr.push(id);
          instructor.findOneAndUpdate({"instructorEmail":data.Instructor},{$set:{classIDs: arr}});
  })
  </code></pre>