节点中的mongodb聚合查询？

Question

我正在尝试在mongoDB中使用非常基本的聚合查询，然后将其应用于节点。

我正在使用mongoDB中的这个数据集：

> db.grades.find().pretty()
{
        "_id" : ObjectId("5276c13ddc170c04635c1eaf"),
        "student" : "Joe",
        "assignment" : "hw2",
        "grade" : 80
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb0"),
        "student" : "Joe",
        "assignment" : "hw3",
        "grade" : 85
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb1"),
        "student" : "Joe",
        "assignment" : "exam",
        "grade" : 100
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb3"),
        "student" : "Steve",
        "assignment" : "hw2",
        "grade" : 90
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb4"),
        "student" : "Steve",
        "assignment" : "hw3",
        "grade" : 100
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb5"),
        "student" : "Steve",
        "assignment" : "exam",
        "grade" : 100
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb7"),
        "student" : "Amanda",
        "assignment" : "hw2",
        "grade" : 90
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb8"),
        "student" : "Amanda",
        "assignment" : "hw3",
        "grade" : 80
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb9"),
        "student" : "Amanda",
        "assignment" : "exam",
        "grade" : 100
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eba"),
        "student" : "Susan",
        "assignment" : "hw1",
        "grade" : 100
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1ebb"),
        "student" : "Susan",
        "assignment" : "hw2",
        "grade" : 90
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1ebc"),
        "student" : "Susan",
        "assignment" : "hw3",
        "grade" : 85
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1ebd"),
        "student" : "Susan",
        "assignment" : "exam",
        "grade" : 80
}
{
        "_id" : ObjectId("5276c13cdc170c04635c1eae"),
        "student" : "Joe",
        "assignment" : "hw1",
        "grade" : 90,
        "date_returned" : ISODate("2013-11-03T23:44:18.774Z")
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb2"),
        "student" : "Steve",
        "assignment" : "hw1",
        "grade" : 80,
        "date_returned" : ISODate("2013-11-03T23:44:31.496Z")
}
{
        "_id" : ObjectId("5276c13ddc170c04635c1eb6"),
        "student" : "Amanda",
        "assignment" : "hw1",
        "grade" : 100,
        "date_returned" : ISODate("2013-11-04T00:59:49.446Z")
}
>


> db.grades.find().pretty().count()
16
>

现在，这是我想在节点中运行的聚合查询：

> db.grades.aggregate([{$group: {_id: "$student"}}])
{
        "result" : [
                {
                        "_id" : "Susan"
                },
                {
                        "_id" : "Amanda"
                },
                {
                        "_id" : "Steve"
                },
                {
                        "_id" : "Joe"
                }
        ],
        "ok" : 1
}

这是我在我正在使用的节点中的尝试。我正在使用快递来设置路线。

app.get('/Work5_6', function(req, res){

    // connect to collection
    db.collection('grades').aggregate( {$group: {_id: "$student"}} , function (err, result) {   

        //if there's an error 
         if(err) throw err;

        //if there is no more results
        if (!result) {
        console.dir("xNo Document found!");

            res.send('This is a test page...',200);

        }

        //...do stuff here to your doc
        console.dir("value of result is:");
        console.dir(result);

    }); 

});

这是我进入控制台的结果：

'value of result is:'
[]

如何访问从节点中的聚合查询返回的“result”值？感谢一些帮助。