title: Access other fields in getter
body: <p>I'm trying to make it that every time I get <code>subtotal</code>, it adds 2 other fields, <code>productTotal</code>, and <code>tax</code>.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>const cost = new mongoose.Schema({
      payment: {
          productTotal: Number,
          tax: Number,
          subtotal: Number, // (productTotal + tax)
      }
  });
  
  
  const Cost = mongoose.model('Cost', cost);
  </code></pre>
  
  <p>How can I add 2 feilds from the same schema when getting a different field?</p>