title: MongoDB aggregation: Project separate document fields into a single array field
body: <p>I have a document like this:</p>
  
  <pre><code>{fax: '8135551234', cellphone: '8134441234'}
  </code></pre>
  
  <p>Is there a way to project (without a group stage) this document into this:</p>
  
  <pre><code>{
      phones: [{
          type: 'fax',
          number: '8135551234'
      }, {
          type: 'cellphone',
          number: '8134441234'
      }]
  }
  </code></pre>
  
  <p>I could probably use a group stage operator for this, but I'd rather not if there's any other way, because my query also projects several other fields, all of which would require a <code>$first</code> just for the group stage.</p>
  
  <p>Hope that's clear. Thanks in advance!</p>