如何对子文档+ MongoDB + Javascript中的字段求和？

Question

Could you kindly help me with this query? How do I sum the qty field in the below document? I have written a piece of code which seems to work when you're accessing price , age etc but it seems to not work well when you are accessing sub documents. Here in this case total = 0`.

Document:

{
"_id" : ObjectId("50a8240b927d5d8b5891743c"),
"cust_id" : "abc123",
"status" : "A",
"price" : 75,
"age" : 24,
"ord_date" : "02/02/2012",
"items" : [ 
    {
        "sku" : "mmm",
        "qty" : 5,
        "price" : 2.5
    }, 
    {
        "sku" : "nnn",
        "qty" : 500,
        "price" : 50
    }
]

}

CODE:

 exports.test = function(collection, callback) {

 MongoClient.connect(url, function(err, db) {
    assert.equal(null, err);
    db.collection(collection).aggregate(

        {
            $match : { "price" : { $gt: 70, $lt: 90 }}
        },
        { 
            $group : { total: { $sum: "$items.qty" }}
        },
    function(err, result) {
        console.log("result",result);
        if (result) {
            callback(result);
        } else {
            callback(false);
        }  
    });
});
}

Appreciate your help!

Thank you,

Answer 1

You need to add {$unwind items} before grouping. That should do the trick.

Answer 2

This should work if used from the command prompt: -

db.inventory.aggregate([
{$unwind: "$items"},
{$match: {"items.price" : { $gt: 70, $lt: 90 }}},
{$group:{_id:"$_id","total": {$sum:"$items.price"}}}
])

Answer 3

try this with unwind function.

exports.test = function(collection, callback) {

 MongoClient.connect(url, function(err, db) {
    assert.equal(null, err);
    db.collection(collection).aggregate(
        {$unwind: "$items"} ,
        {
            $match : { "items.price" : { $gt: 70, $lt: 90 }}
        },
        { 
            $group : { _id:null,total: { $sum: "$items.qty" }}
        },
    function(err, result) {
        console.log("result",result);
        if (result) {
            callback(result);
        } else {
            callback(false);
        }  
    });
});
}