title: Add property to mongoose query result in nested queries
body: <p>I have two models: <em>Competition</em> and <em>Like</em>. there is an one-to-many relationship between them. Now I want to retrieve all competitions and add one property <strong>isLiked</strong> to each of competitions if there is a row in Like table.</p>
  
  <p>This is competition model query and result :</p>
  
  <p>Query:</p>
  
  <pre><code>Competition.find().lean().exec(function (err, competitions) {
      if (err) {
          console.log(err);
      }
      else {
          res.json(competitions);
      }
  });
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>[
      {
          "_id": "5b0a677ab8ceab2132a6491e",
          "mediaType": "MOV",
          "isScoreViewable": true,
          "title": "Liverpool vs RealMadrid",
          "__v": 0
      },
       ...
  ]
  </code></pre>
  
  <p>and I want something like this:</p>
  
  <pre><code>[
      {
          "_id": "5b0a677ab8ceab2132a6491e",
          "mediaType": "MOV",
          "isScoreViewable": true,
          "title": "Liverpool vs RealMadrid",
          "__v": 0,
          "isLiked": true &lt;=== Must be add
      },
       ...
  ]
  </code></pre>
  
  <p>I tried below code but <strong>isLiked</strong> property doesn't add to result: </p>
  
  <pre><code>Coin.find().lean().exec(function (err, competitions) {
      if (err) {
          console.log(err);
      }
      else {
          Like.find({ iid: req.params.id }).lean().exec(function (err, likes) {
              if (err) {
                  console.log(err);
              }
              else {
                  competitions.forEach(function(competition) {
                      likes.forEach(function(like) {
                          competition.isLiked = like.competition_id == competition._id;
                      });
                  });
              }
          });
          return res.json(competitions);
      }
  });
  </code></pre>