Database
[{"prod_id":"1022","whole_price":"10","decimal_price":"50"},{"prod_id":"1022","whole_price":"10","decimal_price":"a"}]
Query
db.collection.aggregate([{"$project":{"_id":0,"prod_id":1,"complete_price":{"$convert":{"input":{"$concat":["$whole_price",".","$decimal_price"]},"to":"double","onError":null,"onNull":null}}}}])