[{"status":true,"data":[{"_id":"61a2a9680b122bc154cbd6af","ownerName":"Ajay","mobile":878787878,"designation":"IT","gender":"Male","age":26,"carDetails":[{"carName":"BUY","color":"blue","sheets":8,"avgSpeed":105,"price":850000,"model":"C110"},{"carName":"GTR","color":"blue","sheets":4,"avgSpeed":105,"price":98000,"model":"G120"}]},{"_id":"61a2a9680b122bc154cbd6af","ownerName":"John","mobile":878787878,"designation":"IT","gender":"Male","age":26,"carDetails":[{"carName":"BUY","color":"blue","sheets":8,"avgSpeed":105,"price":850000,"model":"C110"}]}]}]

db.collection.aggregate([{"$unwind":{"path":"$data"}},{"$set":{"data.total_car":{"$size":"$data.carDetails"},"data.carDetails":"$$REMOVE","_id":"$$REMOVE"}},{"$set":{"ownerDetails":"$data","data":"$$REMOVE"}}])