MongoDB Aggregation Sum on Objects in Array -

i've seen lot of answers on how sum properties of objects in arrays within array, i'm trying sum individual properties on object in array across documents. example, given document structure:

{    "id": 1,    "stats": [      {        "number": 100,        "year": 2014      },      {        "number": 200,        "year": 2015      } ] },   {    "id": 2,    "stats": [      {        "number": 50,        "year": 2014      },      {        "number": 75,        "year": 2015      } ] } 

the desired output be:

{    "stats": [      {        "number": 150,        "year": 2014      },      {        "number": 275,        "year": 2015      } } 

i don't want sum number property of 2014 , 2015, want sum across 2014 both documents.

db.test.aggregate([    {  $unwind: "$stats" },    {         $group: {             _id:"$stats.year",             number:{$sum:"$stats.number"}         }     },     {          $group: {           _id: 0,             stats:{ $push:  {year:"$_id",number:"$number"}}         }     },     {           $project:{stats:1,_id:0}     } ])