How to multiply NumberDecimal values in mongodb - mongodb

Question

I have the following structure:
{
"_id": "5d0118f0f57a282f89bc5f71",
"product": {
"_id": "5cfed37375a13067dd01ddb7",
"name": "My product",
"description": "My description",
"purchased_amount": 15,
"unit_price_mex": "45",
"unit_price_to_sell": "5",
"travel": "5cf58713d6f7f1657e2d8302",
"__v": 0,
"id": "5cfed37375a13067dd01ddb7"
},
"client": {
"_id": "5cf1778efffb651fad89d8b6",
"name": "Client name",
"description": "",
"__v": 0
},
"purchased_amount": 3,
"fch": "13/6/2019",
"__v": 0
},
{
"_id": "5d0151afda1a446008f1817b",
"product": {
"_id": "5cfed1995eaf2665c45efd82",
"name": "Camisa",
"description": "Camisas buenas",
"purchased_amount": 10,
"unit_price_mex": "100",
"unit_price_to_sell": "15",
"travel": "5cf56b04462a865264fabb9d",
"__v": 0,
"id": "5cfed1995eaf2665c45efd82"
},
"client": {
"_id": "5cf1778efffb651fad89d8b6",
"name": "Randy",
"description": "El que trabaja aqui",
"__v": 0
},
"purchased_amount": 34,
"fch": "12/6/2019",
"__v": 0
},
Where client and product are of type ObjectId. This is the Schema:
Client Model
var mongoose = require('mongoose');
const mongoosePaginate = require('mongoose-paginate-v2');
var clientSchema = new mongoose.Schema({
name: String,
description: String
}).plugin(mongoosePaginate);
var Client = mongoose.model('Client', clientSchema);
module.exports = Client;
Product Model
var mongoose = require('mongoose');
const mongoosePaginate = require('mongoose-paginate-v2');
var productSchema = new mongoose.Schema({
name: String,
description: String,
purchased_amount: Number,
unit_price_mex: mongoose.Schema.Types.Decimal128,
unit_price_to_sell: mongoose.Schema.Types.Decimal128,
travel: { type: mongoose.Schema.Types.ObjectId, ref: 'Travel' }
}).plugin(mongoosePaginate);
productSchema.set('toJSON', {
getters: true,
transform: (doc, ret) => {
if (ret.unit_price_mex) {
ret.unit_price_mex = ret.unit_price_mex.toString();
}
if ( ret.unit_price_to_sell ) {
ret.unit_price_to_sell = ret.unit_price_to_sell.toString();
}
}
})
var Product = mongoose.model('Product', productSchema);
module.exports = Product;
I need to get the multiplication sum of purchased_amount by product.unit_price_to_sell. My code is the following but always returns 0. Apparently, "$product.unit_price_to_sell" does not return the decimal value.
var aggregate = InvoiceModel.aggregate([
{ $match: { client: mongoose.Types.ObjectId( id ) } },
{ $group: { _id: null, total: { $sum: { $multiply: [ "$purchased_amount", "$product.unit_price_to_sell" ] } } } }
]);
InvoiceModel.aggregatePaginate(aggregate, {}, (error, aggs) => {
InvoiceModel.paginate({ client: id },{ page, limit, populate: 'client product' }, (err, value) => {
return res.status(200).send({
results: value.docs,
totalPages: value.totalPages,
totalDocs: value.totalDocs,
purchase_amount_total : aggs.docs[0].total
})
})
})

Answer 1

MongoDB cannot use string values in arithmetic expressions. You must either store the values using their numeric non-string representations, or you must use an aggregation operator like $toDecimal to convert the values to their numeric representations first.
Modifying your $group stage to something like the following should work:
{ $group: { _id: null, total: { $sum: { $multiply: [ "$purchased_amount", { $toDecimal: "$product.unit_price_to_sell" } ] } } }
Please note, however, that this will only work for MongoDB versions >= 4.0. If you're using an older version of MongoDB, you will either need to upgrade it to at least version 4.0 or begin converting your existing values from strings to numbers.