title: Spring MongoTemplate Update Merge
body: <p>Using spring's mongoTemplate or otherwise how do I perform a simple merge on an existing mongo document?</p>
  
  <p>When I say merge I want the following to happen:</p>
  
  <ol>
  <li>If a field in modifier document exists on the backend version of the document, then update it with the new value in the modifier.</li>
  <li>If a field in the modifier document does NOT exist on the backend version, then add the field.</li>
  <li>Leave all other fields on the Backend document alone.</li>
  </ol>