Can not get $sum in aggregate function in MongoDB - mongodb

Question

Here's my data format :
"request" : {
"_id" : 1003,
"user" : {
"username" : "",
"password" : "",
"_id" : 1111,
"gender" : "male",
},
"request" : {
"merchantid" : "TA456",
"txnAmount" : 18000,
"fee" :0,
"IssuerID" : "18801111",
"bankID" : "888888",
}
},
"confirmation" : true
}`
I can not get the sum of 'request.request.txnAmount' group by 'request.user._id'.
Here's my query:
db.getCollection('megabank_response').aggregate(
[ { $group : {
_id : "request.user._id",
totalPrice: { $sum: "request.request.txnAmount"},
count: { $sum: 1 }
}
}
])
And get the result :
{
"_id" : "request.user._id",
"total Price" : 0,
"count" : 4.0
}
Can anyone help me why my query cannot calculate the sum?

Answer 1

You seem to missing the $ symbol in the group aggregation function.
db.megabank_response.aggregate([{
$group: {
_id: "request.user._id",
ss: {
$sum: "request.request.txnAmount"
}
}
}]);
gives: { "_id" : "request.user._id", "ss" : 0 }
But the correct answer is:
db.megabank_response.aggregate([{
$group: {
_id: "$request.user._id",
ss: {
$sum: "$request.request.txnAmount"
}
}
}]);

Answer 2

I've managed to figured out the query using answer from this source.
However, my query is stil a little bit too messy. Any ways to optimize it?
db.getCollection('megabank_response').aggregate([
{
"$match": {
"request.user._id": 1111
}
},
{
"$group":{
_id : "request.user._id",
totalPrice: { $sum: '$request.request.txnAmount'},
count: { $sum: 1 }
}
}
])
Result:
{
"_id" : "request.user._id",
"totalPrice" : 62000,
"count" : 3.0
}

Answer 3

Try:
db.getCollection('megabank_response').aggregate([{
$group: {
_id : "request.user._id",
totalPrice: { $sum: "$request.request.txnAmount"},
count: { $sum: 1 }
}
}])

Answer 4

You can also use totalPrice: { $sum: "$txnAmount"} to display the sum of your transaction amount. I was working on the GET request of Total Sales in the e-shop that I developed, and I had faced the same issue. Just for your reference, here is my code:
//GET total sales data for the ADMIN
router.get('/get/totalsales', async(req, res) => {
const totalSales = await Order.aggregate([
{ $group: { _id: null, totalSales : { $sum: '$totalPrice' }}} // Use $totalPrice instead of totalPrice
])
if(!totalSales) {
return res.status(400).send('The sales cannot be generated')
}
res.send({totalSales: totalSales})
})
Hope this helps :)