title: MongoDB change stream display only selected fields
body: <p>I am trying to understand change stream in MongoDB.</p>
  
  <p>I am trying to display only some fields of updated document.</p>
  
  <p>So I did:</p>
  
  <pre><code>option={ 'full_document':'updateLookup' }
  collection.watch([{"$match"  : { "operationType" : "update" }}] , **option)
  </code></pre>
  
  <p>which is working.</p>
  
  <p>Now I want to display only some fields.</p>
  
  <p>I tried:</p>
  
  <pre><code>collection.watch([{"$match"  : { "operationType" : "update" }},{"$project":{"_id":1}}] , **option)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>collection.watch([{"$match"  : { "operationType" : "update" }}],option).aggregate({"$project":{"_id":1}})
  </code></pre>
  
  <p>but neither of them works.</p>
  
  <p>How can I display only selected fields?</p>