title: MongoDB multi level group
body: <p>I have some input data :</p>
  
  <pre><code>Brand   | Model   | Number
  Peugeot | 208     | 1
  Peugeot | 4008    | 2
  Renault | Clio    | 3
  Renault | Megane  | 4
  </code></pre>
  
  <p>I would like to get both :</p>
  
  <ul>
  <li><p>the sum for each brand </p></li>
  <li><p>the global sum</p></li>
  </ul>
  
  <p>Here is my expected output :</p>
  
  <pre><code>Brand   | Number
  Peugeot | 3
  Renault | 7
  Total   | 10
  </code></pre>
  
  <p>I think I have to create two <code>$group</code> operations and set Total with <code>$literal</code>.
  What is the right way to do so ?</p>