[{"key":1},{"key":2},{"key":3},{"key":4},{"key":50},{"key":6},{"key":7}]

db.collection.aggregate([{$match:{key:{$in:[1,3,5,7]}}},{$facet:{result:[],notFound:[{$group:{_id:null,key:{$addToSet:"$key"}}},{$project:{_id:0,key:{$setDifference:[[1,3,5,7],"$key"]}}}]}}])