Database
[{_id:"2",poDetails:{material:"300000114",raisedGrnQuantity:"20"}},{_id:"1",poDetails:{material:"300000114",raisedGrnQuantity:"15"}},{_id:"3",poDetails:{material:"910000029",raisedGrnQuantity:"10"}},{_id:"62de30ea1ad8470012b9dc78",poDetails:{material:"910000029",raisedGrnQuantity:"5"}}]
Query
db.collection.aggregate([{$group:{_id:"$poDetails.material",raisedGrnQuantity:{$sum:{"$toInt":"$poDetails.raisedGrnQuantity"}}}}])