mongo query group by day and count

mongo_query_group_by_day_and_count.js

db.sales.aggregate([
     {
      $group : {
        _id : {//Agruppated mandatory field (doesnt mean the ObjectId of document)
    		    'year' : { '$year': "$createdAt"},
	          'month' : { '$month': "$createdAt"},
        		'day' : { '$dayOfMonth': "$createdAt"}
        }, 
        count:{
          $sum:1
          }
        }
     }
])