title: Mongoose: Upsert document with $sum
body: <p>Say I have a document that looks like this:</p>
  
  <pre><code>var TestSchema = new Schema({
      _id: String,
      userId: Number,
      date: Date,
      usage: Object
  });
  </code></pre>
  
  <p>When I insert something into the database, that document can look like:</p>
  
  <pre><code>{
      userId: 10,
      date: 2015-11-12,
      usage: {
          foo: 12,
          bar: 5
      }
  }
  </code></pre>
  
  <p>Now is it possible to upsert this document, where usage uses the $sum of the new document and the old document?</p>
  
  <p>So when I do <code>findOneAndUpdate</code>, I basically only want to add up the usage.</p>