MongoDB querying from Aggregation to Group

问题:

I have this following query in MongoDB:

var subquery = {"shipdate" : { 
"$gte" : 19960101, 
"$lt" : 19960401 } 
};
var eachsupp = db.lineitems.aggregate([
{ $match : subquery },
{ $project : {
    "_id" : 0,
    "revenue" : {$multiply : ["$extendedprice", {$subtract : [1, "$discount"] }] },
    "supplier_no" : "$partsupp.supplier.suppkey",
    "name" : "$partsupp.supplier.name",
    "address" : "$partsupp.supplier.address",
    "phone" : "$partsupp.supplier.phone" }},
{ $group : {
    _id : "$supplier_no",
    total_revenue : { $sum : "$revenue" },
    name : {$first : "$name"},
    address : {$first : "$address"},
    phone : {$first : "$phone"} }}
]);
eachsupp.forEach(function(document){db.tmp.insert(document)});
db.tmp.find().sort({total_revenue : -1}).limit(1);

But for academical purposes I need this query using .group(). Something like this

db.lineitems.group(...);

I'm starting like this but I don't know how to go with the next step

var red = function(doc, out){
out.supplier_no
out.revenue_calc = doc.extendedprice*(1-doc.discount);
};

db.lineitems.group({
key : "revenue",
cond : subquery,
initial : {revenue_calc : 0},
reduce : red,
finalize : fin
});