Database
[{"fk":1,"name":"Adam"},{"fk":1,"name":"George"},{"fk":1,"name":"Michael"},{"fk":2,"name":"George"},{"fk":2,"name":"Adam"},{"fk":2,"name":"Michael"}]
Query
db.collection.aggregate([{$match:{fk:{$in:[1,2]}}},{$sort:{fk:1,name:-1}},{$group:{_id:"$fk",items:{$push:"$$ROOT"}}},{$project:{items:{$slice:["$items",2]}}}])