title: how to convert number to month in mongo aggregation
body: <p>I am trying this mongo aggregation I got the output but my requirement is to get the month name. I'm trying diffrent types of aggregation still not getting requirement output, Can anyone please help me.</p>
  
  <pre><code>    db.collection.aggregate([ 
              { $match: {
                  CREATE_DATE: {
                      $lte:new Date(),
                      $gte: new Date(new Date().setDate(new 
                          Date().getDate()-120)
                      )
                  }
              } },
              { $group: {
                  _id:{ 
                      month: { $month: "$CREATE_DATE" },
                      year: { $year: "$CREATE_DATE" }
                  },
                  avgofozone:{$avg:"$OZONE"}
              } },
              { $sort:{ "year": -1 } },
              { $project: {
                  year: '$_id.year',
                  avgofozone: '$avgofozone',
                  month: '$_id.month',_id:0
              } }
         ])
  </code></pre>
  
  <p><strong>Output as of now:</strong></p>
  
  <pre><code>/* 1 */
      {
         "year" : 2018,
         "avgofozone" : 16.92,
         "month" : 4
      }
  
  /* 2 */
      {
         "year" : 2017,
         "avgofozone" : 602.013171402383,
         "month" : 12
      }
  </code></pre>
  
  <p><strong>Expected output:</strong></p>
  
  <pre><code>/* 1 */
      {
          "year" : 2018,
          "avgofozone" : 16.92,
          "month" : APRIL
      }
  
  /* 2 */
     {
         "year" : 2017,
         "avgofozone" : 602.013171402383,
         "month" : DECEMBER
     }
  </code></pre>