title: How can i add new field to Mongo aggregate result?
body: <p>I have following mongodb aggregate query,that group by date (1-05-2017  to  1-13-2017)</p>
  
  <pre><code>CampaignActivity.aggregate([
          {
          "$match": {
              "$and":[{updatedAt: {$lt: new Date('1-13-2017')}},{updatedAt: {$gte: new Date('1-05-2017')}}]
          }
      },
      {
          "$project" :
          {
             _id : 0,
             "datePartDay" : {"$concat" : [
                 {"$substr" : [{"$dayOfMonth" : "$updatedAt"}, 0, 2]}, "-",
                 {"$substr" : [{"$month" : "$updatedAt"}, 0, 2]}, "-",
                 {"$substr" : [{"$year" : "$updatedAt"}, 0, 4]}
            ] },
            "isdelivered": {"$cond": { if: { $eq: [ "$delivered", true ] }, then: 1, else: 0 }},
            "isclicked": {"$cond": { if: { $eq: [ "$clicked", true ] }, then: 1, else: 0 }},
            "isunsubscribe": {"$cond": { if: { $eq: [ "$unsubscribed", true ] }, then: 1, else: 0 }}
  
          }
      },
  
      { "$group" :
          { "_id" : "$datePartDay",
            "delivered" : { "$sum" :'$isdelivered' }, 
            "clicked" : { "$sum" :'$isclicked' }, 
            "unsubscribed" : { "$sum" :'$isunsubscribe' } 
          }
      }
      ],function (err, result) {
          if (err) {
              console.log(err);
          } else {
              console.log(result);
          }
      });
  </code></pre>
  
  <p>Aggregate result :</p>
  
  <pre><code> [
         {
           "_id": "12-1-2017",
           "delivered": 0,
           "clicked": 1,
           "unsubscribed": 1
         },
         {
           "_id": "11-1-2017",
           "delivered": 2,
           "clicked": 1,
           "unsubscribed": 0
         }
     ]
  </code></pre>
  
  <ul>
  <li><p><strong>Is it possible to add new fields to result that no exist in collection something like "date"?</strong></p></li>
  <li><p><strong>Is it possible to add missing date fields with dummy data?</strong></p></li>
  </ul>