Database
[{name:"Abhishek",Roll_no:null,hobby:"stackoverflow"},{name:null,Roll_no:1,hobby:null},{name:"someOne",Roll_no:1,hobby:"sometHIng"},{name:"someOne",Roll_no:2,hobby:null},{name:null,Roll_no:2,hobby:"not sure"},{name:null,Roll_no:null,hobby:null}]
Query
db.collection.aggregate([{$project:{doc:{$arrayToObject:{$filter:{input:{$objectToArray:"$$ROOT"},cond:{$ne:["$$this.v",null]}}}}}},{$replaceRoot:{newRoot:"$doc"}}])