title: How to add new fields to existing document
body: <p>In the <code>coupes</code> collection in my database, an example record looks like this: </p>
  
  <pre><code>db.coupes.findOne()
  {
      "_id" : ObjectId("5949834be916d79e033500c6"),
      "rank" : 10,
      "brand" : "Toyota",
      "model" : "86SE"
  }
  </code></pre>
  
  <p>However, I'd like to update it so that it looks like the following:</p>
  
  <pre><code> {
  "_id" : ObjectId("5949834be916d79e033500c6"),
  "rank" : 10,
  "brand" : "Toyota",
  "model" : "86SE", 
  "engineSpec" : {"cylinderConfiguration": "H-4", "transmission" : "6 speed auto"}
   }
  </code></pre>
  
  <p>How might I accomplish this?</p>