MongoDB aggregation projection on collection -

i have collection full of documents this.

{     "_id" : objectid("5696db8a049c1bb2ecaaa10f"),     "resultsets" : [         {             "headers" : [ "a", "b" ],             "data" : [                 [ 0, 1 ]             ]         },          {             "headers" : [ "c", "d" ],             "data" : [                 [ 2, 3 ]             ]          }    ] } 

i use aggregation project document just.

{ c: 2, d: 3 } 

just wondering best way be.

if using mongodb 3.2 $arrayelemat go long way in getting desired aggregation output dealing arrays.

run following aggregation pipeline, uses $arrayelemat operator return desired values in new keys:

var pipeline = [     {         "$match": { "resultsets.headers": { "$in": [ "c", "d" ] } }     },     { "$unwind": "$resultsets" },     {         "$match": { "resultsets.headers": { "$in": [ "c", "d" ] } }     },     {         "$project": {             "_id": 0,             "c": { "$arrayelemat": [ "$resultsets.data", 0 ] },             "d": { "$arrayelemat": [ "$resultsets.data", 1 ] }         }     } ]  db.collection.aggregate(pipeline);