title: $push on the basis $cond in mongodb?
body: <p>I have <code>Review</code> schema having <code>likeBy</code> array contains the id of the users who likes the review... 
  I have to query it like 
  case 1: If id exist the pull it.
  case 2: If id does not exist then push it.
  Is it possible to do in a single query???</p>
  
  <p>Review Schema    </p>
  
  <pre><code>{  
  "_id" : ObjectId("5aa0f4aeb5cbb2313276cccc"),
  "__v" : 0,
  "likeBy" : [ ]
  }
  </code></pre>