[{"_id":1,"groupID":1,"content":"content 1","createAt":1},{"_id":2,"groupID":1,"content":"content 2","createAt":2},{"_id":3,"groupID":2,"content":"content 3","createAt":3},{"_id":4,"groupID":2,"content":"content 4","createAt":4},{"_id":5,"groupID":2,"content":"content 5","createAt":5},{"_id":6,"groupID":2,"content":"content 6","createAt":6}]

db.collection.aggregate([{$sort:{createAt:1}},{$group:{_id:"$groupID",last:{$last:"$$ROOT"}}},{$replaceRoot:{newRoot:"$last"}}])