Mongodb $sum with Subarray list item values - json

Question

I am trying to calculate total number of products in my order which was actually in sub-array.
Facing trouble with $sum while trying to calculate.
My Attempt
db.getCollection('orders').aggregate([
{$match:{'order_no':'GHT19'}},
{$unwind:"$products_list"}
])
My Result
{
"_id" : ObjectId("57838a3a3628f76b52511ffe"),
"order_no" : "GHT19",
"origin_port" : "Vizag Port",
"destination_port" : "Hankou Port"
"products_list" : {
"box_no" : "1",
"product_name" : "Mobile",
"qty" : "5"
}
}
{
"_id" : ObjectId("57838a3a3628f76b52511ffe"),
"order_no" : "GHT19",
"origin_port" : "Vizag Port",
"destination_port" : "Hankou Port"
"products_list" : {
"box_no" : "2",
"product_name" : "Television",
"qty" : "2"
}
}
{
"_id" : ObjectId("57838a3a3628f76b52511ffe"),
"order_no" : "GHT19",
"origin_port" : "Vizag Port",
"destination_port" : "Hankou Port",
"products_list" : {
"box_no" : "3",
"product_name" : "Radio",
"qty" : "2"
}
}
While trying to do SUM like below
db.getCollection('orders').aggregate([
{$match:{'order_no':'GHT19'}},
{$unwind:"$products_list"},
{$group: {
_id: '$products_list.box_no',
"total_products": {$sum: "$products_list.qty" }
}
}
])
I am getting ZERO result. Please see the below result.
{
"_id" : "2",
"total_products" : 0
}
{
"_id" : "3",
"total_products" : 0
}
{
"_id" : "1",
"total_products" : 0
}
Please find the best solution for my issue.

Answer 1

Your agreggation is correct, the problem is that qty field is a String. Try changing it to "qty":3. That should work

Answer 2

You can compute $sum only with numeric value : https://docs.mongodb.com/manual/reference/operator/aggregation/sum/
You can convert all your products_list.qty to numeric with :
db.device.find().forEach(function(data) {
data.products_list.forEach(function(data2) {
db.device.update({
"_id": data._id,
"products_list.box_no": data2.box_no
}, {
"$set": {
"products_list.$.qty": parseInt(data2.qty)
}
});
});
})
And then execute your aggregation