How to get MongoDB _id of an inserted document - node.js

Question

I want to create a document in my MongoDB database and take the _id of the new document.
This is what I'm doing:
const mongoose = require("mongoose");
const billingSchema = require("./models/billing");
const { ObjectId } = require("bson");
const { MongoClient } = require("mongodb");
const mongouri = "***";
var connection = mongoose.createConnection(mongouri);
var Bills = connection.model("Fatturazione", billingSchema, "Fatturazione");
exports.createBill = (b) => {
return new Promise((resolve, reject) => {
Bills.Create(b, function (err) {
if (err) {
reject(err);
} else {
console.log(mongoose.Types.ObjectId(b._id));
resolve();
}
});
});
};
and this is my Schema:
const mongoose = require("mongoose");
const Schema = mongoose.Schema;
//schema define the structure of the document
const billingSchema = new Schema({
data_fatturazione: {
type: Date,
required: true,
},
data_saldo: {
type: Date,
required: false,
},
totale: {
type: Number,
required: false,
},
pagato: {
type: Boolean,
required: false,
},
});
module.exports = billingSchema;
In the console.log() I want to print the _id of the last inserted document but it prints a non-existing id (it doesn't correspond to the _id of the last created document in the database). I also tried without using mongoose.Types.ObjectId() but it prints undefined. I don't understand where is the problem.
I call the function createBill() in another js file, passing an object with the correct fields.

Answer 1

You are trying to get the _id of argument b, which is passed to your createBill, which is logically undefined. Instead you must get the _id from a result of Bill.create, mongoose callbacks take 2 arguments as #Joe mentioned in the comments, so your code must look like this:
exports.createBill = (b) => {
return new Promise((resolve, reject) => {
Bills.Create(b, function (err, result) {
if (err) {
reject(err);
} else {
console.log(result._id);
resolve(result);
}
});
});
};