Database
[{"_id":1,"item":"item1","quantity":2,"date":ISODate("2014-01-01T08:00:00Z")},{"_id":2,"item":"item2","quantity":1,"date":ISODate("2014-02-03T09:00:00Z")},{"_id":3,"item":"item2","quantity":5,"date":ISODate("2014-02-03T09:05:00Z")},{"_id":4,"item":"item1","quantity":10,"date":ISODate("2014-02-15T08:00:00Z")},{"_id":5,"item":"item2","quantity":10,"date":ISODate("2014-02-15T09:05:00Z")}]
Query
db.collection.aggregate([{$sort:{date:-1}},{$group:{_id:"$item",root:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$root"}}])