Mongoose SUM get stacked - node.js

Question

I'm trying to make trivial SUM on mongoDB to count number of prices for single client.
My collection:
{"_id":"5d973c71dd93adfbda4c7272","name":"Faktura2019006","clientId":"5d9c87a6b9676069c8b5e15b","expiration":"2019-10-02T01:11:18.965Z","price":999999,"userId":"123"},
{"_id":"5d9e07e0b9676069c8b5e15d","name":"Faktura2019007","clientId":"5d9c87a6b9676069c8b5e15b","expiration":"2019-10-02T01:11:18.965Z","price":888,"userId":"123"}
What I tried:
// invoice.model.js
const mongoose = require("mongoose");
const InvoiceSchema = mongoose.Schema({
_id: String,
name: String,
client: String,
userId: String,
expiration: Date,
price: Number
});
module.exports = mongoose.model("Invoice", InvoiceSchema, "invoice");
and
// invoice.controller.js
const Invoice = require("../models/invoice.model.js");
exports.income = (req, res) => {
console.log("Counting Income");
Invoice.aggregate([
{
$match: {
userId: "123"
}
},
{
$group: {
total: { $sum: ["$price"] }
}
}
]);
};
What happen:
When I now open a browser and code above is being called, I get console log 'Counting Income' in terminal however in browser it's just loading forever and nothing happen.
Most likely I just miss some stupid minor thing but I'm trying to find it out for quite a long time without any success so any advise is welcome.

Answer 1

The reason that the controller never finishes is because you are not ending the response process (meaning, you need to use the res object and send something back to the caller).
In order to get the aggregate value, you also need to execute the pipeline (see this example).
Also, as someone pointed out in the comments, you need to add _id: null in your group to specify that you are not going to group by any specific field (see the second example here).
Finally, in the $sum operator, for what you're trying to do, you just need to remove the array brackets since you only want to sum on a single field (see a few examples down here).
Here is the modified code:
// invoice.controller.js
const Invoice = require("../models/invoice.model.js");
exports.income = (req, res) => {
console.log("Counting Income");
Invoice.aggregate([
{
$match: {
userId: "123"
}
},
{
$group: {
_id: null,
total: { $sum: "$price" }
}
}
]).then((response) => {
res.json(response);
});
};
Edit for your comment about when an empty array is returned.
If you want to always return the same type of object, I would control that in the controller. I'm not sure if there is a fancy way to do this with the aggregate pipeline in mongo, but this is what I would do.
Invoice.aggregate([
{
$match: {
userId: "123"
}
},
{
$group: {
_id: null,
total: { $sum: "$price" }
}
},
{
$project: {
_id: 0,
total: "$total"
}
}
]).then((response) => {
if (response.length === 0) {
res.json({ total: 0 });
} else {
// always return the first (and only) value
res.json(response[0]);
}
});
Here, if you find a userId of 123, then you would get this as the return:
{
"total": 1000887
}
But if you change the userId to, say, 1123 which doesn't exist in your db, the result will be:
{
"total": 0
}
This way, your client can always consume the same type of object.
Also, the reason I put the $project pipeline stage in there was to suppress the _id field (see here for more info).