title: Use only limited keys from $lookup aggregation
body: <p>I want to filter data after $lookup aggregation.</p>
  
  <p>Now, I want the only value of location key from child collection, not the whole document. Also if I need the specific key in location i.e zone_id  What should I do? Please help.</p>
  
  <p>I am using below query</p>
  
  <p>// Query</p>
  
  <pre><code>db.parent.aggregate([
          {
            $lookup:
          {
            from: "child",
            localField: "pid",
            foreignField: "pid",
            as: "more"
          } 
         }
  ])
  </code></pre>
  
  <p><strong>// Data
  // Child collection</strong></p>
  
  <pre><code> db.child.insert({
    "pid": 1,
    "name": "Max",
    "title": "Top" 
    "status": 1,
  
    "description": {
      "destination": "jur bagh",
      "community": "abc"
    },
    "location:": {
      "zone_id": "north",
      "city": "jaipur",
      "latitude": "12.121212",
      "longitude": "21.414134"
    }, 
    "created_by": "user_id",
    "modified_by": "user",
    "created_at": "12:00",
    "updated_at": "13:00"
  });
  </code></pre>
  
  <p>// parent collection</p>
  
  <pre><code> db.parent.insert({
    "pid": 1,
    "pname": "PQW",
    "rox": "Labs",
    "status": 1,
    "created_by": "smdcd",
    "modified_by": "pink",
    "created_at": "12:00",
    "updated_at": "13:00"
  });
  </code></pre>
  
  <p>I want result like this </p>
  
  <pre><code>db.parent.insert({
    "pid": 1,
    "pname": "PQW",
    "rox": "Labs",
    "status": 1,
    "created_by": "smdcd",
    "modified_by": "pink",
    "created_at": "12:00",
    "updated_at": "13:00"
  "more" [
  "location:": {
      "zone_id": "north",
      "city": "jaipur",
      "latitude": "12.121212",
      "longitude": "21.414134"
    }
  ]
  
  
  });
  </code></pre>