title: Populate specific fields in $lookup
body: <p>I am using aggregate to group and populate the result like below: </p>
  
  <pre><code> { 
     "$group": {
       "_id": "$userId",
       "projectId": { "$push": "$projectId" }
      }
   },
   { 
    "$lookup": {
       "from": "users",
       "localField": "_id",
       "foreignField": "_id",
        "as": "user"
     }
   },
   {   $unwind:"$user" },
   { 
     "$lookup": {
       "from": "projects",
       "localField": "projectId",
       "foreignField": "_id",
       "as": "projects"
      }
   }
  </code></pre>
  
  <p>But i want to <strong>populate specific fields from that result</strong> For this I tried 
   $project, But it combining projectId into one array and projectName into another array.Below is my result json:</p>
  
  <pre><code>[
    {
      "_id": "5c0a29e597e71a0d28b910aa",
      "projectId": [
          "5c0a2a8897e71a0d28b910ac",
          "5c0a4083753a321c6c4ee024"
      ],
      "user": {
          "_id": "5c0a29e597e71a0d28b910aa",
          "firstName": "Amit"
          "lastName": "kumar",
          "type": "developer",
          "status": "active"
      },
      "projects": [
          {
              "_id": "5c0a2a8897e71a0d28b910ac",
              "skypeId": "",
              "projectName": "LN-PM",
              "status": "ongoing",
              "assignId": "5c0a2a0a97e71a0d28b910ab"
          },
          {
              "_id": "5c0a4083753a321c6c4ee024",
              "skypeId": "",
              "status": "pending",
              "assignId": "5c0a2a0a97e71a0d28b910ab"
          }
      ]
    }
  ]
  </code></pre>
  
  <p>Now i want to get the only <strong>"firstName and _id"</strong> field from <strong>user</strong> field and <strong>"projectName and _id"</strong> field from the <strong>projects</strong> field</p>