title: How to assign the value of a field to another in a single mongo Query
body: <p>Suppose you have a mongo record :</p>
  
  <pre><code>{
      _id : 1234,
      x: 12,
      y: 34
  }
  </code></pre>
  
  <p>I need a query that will update an particular _id in such a way that</p>
  
  <pre><code>x = y;//assign the value of y to x
  y = new Date().getTime();//assign current time to y
  </code></pre>
  
  <p>for a particular _id.currently I am reading the id modifying the entity and updating it.Is this possible to do it in a single query? what would be it's equivalent using Mongoose</p>