Database
[{_id:ObjectId("5b0cf67270e4fa02d31de42e"),name:"xiaoming",time:400,mon:NumberDecimal("100.7645234")},{_id:ObjectId("5b0cf6ad70e4fa02d31de430"),name:"xiaoming",time:0,mon:NumberDecimal("1.23734324")},{_id:ObjectId("5b0d14c870e4fa02d31de436"),name:"xiaoming",time:0,mon:NumberDecimal("1.785233")}]
Query
db.collection.aggregate([{"$group":{_id:"$name",total_count:{"$sum":{"$round":["$mon",2]}}}}])