title: MongoDB insert field to existing document if does not already exist
body: <p>I have a document like this (this is very simplified):</p>
  
  <pre><code>{
      'name' : 'test',
      'age' : 16
  }
  </code></pre>
  
  <p>and also this</p>
  
  <pre><code>{
      'name' : 'test2'
  }
  </code></pre>
  
  <p>I want to set all 'age' to 14 if it does not exist.</p>
  
  <p>How can I do this?</p>