Database
[{"name":"ada","updateTime":"2016-11-25"},{"name":"bob","updateTime":"2016-11-25"},{"name":"ada","updateTime":"2016-11-20"},{"name":"bob","updateTime":"2016-11-20"},{"name":"ada","updateTime":"2016-11-15"},{"name":"bob","updateTime":"2016-11-15"}]
Query
db.collection.aggregate([{$group:{"_id":"$name",updateTime:{$max:"$updateTime"}}}])