title: Mongoose populate data key remove in got data
body: <p>I'm using relation between two collection and I have collection schema like follow.</p>
  
  <pre><code>// Room database Entry.
  {
      "_id" : ObjectId("59b77082f7da4b1b3f64d927"),
      "room_id" : "c3wqqkfndg9cnmi",
      "room_name" : "bcmvnxbcnvsjkfhkjdh",
      "users" : [
          {
              "_id" : ObjectId("59b77082f7da4b1b3f64d926")
          }
      ],
      "__v" : 0
  }
  
  //User Database Entry, Related with above Room entry.
  {
      "_id" : ObjectId("59b77082f7da4b1b3f64d926"),
      "user_id" : "dyr1qlrtso47vi",
  
      "__v" : 0
  }
  </code></pre>
  
  <p>When I pass query for Room Find and populate the data than I found result like follow.</p>
  
  <pre><code>// Query For Room find and Populate.
  room
   .findOne({ room_id: result.data.room_id })
   .populate("users._id")
   .exec(function(err, data){
    });
  
  //Result As Follow
  {
     "_id":"59b77082f7da4b1b3f64d927",
     "room_id":"c3wqqkfndg9cnmi",
     "room_name":"bcmvnxbcnvsjkfhkjdh",
     "__v":0,
     "users":[
        {
           "_id":{
              "_id":"59b77082f7da4b1b3f64d926",
              "user_id":"dyr1qlrtso47vi",
              "__v":0
           }
        }
     ]
  }
  </code></pre>
  
  <p>My Requirement is like follow, I don't want <code>_id</code> as key of object. I need <code>users</code> object like follow.</p>
  
  <pre><code>"users":[
        {
           "_id":{
              "_id":"59b77082f7da4b1b3f64d926",
              "user_id":"dyr1qlrtso47vi",
              "__v":0
           }
        }
     ]
  
  Should be 
  
  "users":[
           {
              "_id":"59b77082f7da4b1b3f64d926",
              "user_id":"dyr1qlrtso47vi",
              "__v":0
           }
       ]
  </code></pre>