Mongodb – post method not posting data in the data base express js / node js

Try updating your code as mentioned below:

const express = require("express");
const bodyParser = require("body-parser");
const mongoose = require('mongoose');
const path = require('path');

const app = express();

app.set('view engine', 'ejs');

app.use(bodyParser.urlencoded({ extended: true }));
app.use(express.static(path.join(__dirname, 'public')));

mongoose.connect("mongodb://localhost:27017/invoice_db", { useNewUrlParser: true });

const fromSchema = new mongoose.Schema({
    title: String,
    number: String,
    email: String
});
const From = mongoose.model("From",fromSchema);

const toSchema = new mongoose.Schema({
    billing_name: String,
    billing_address: String,
    addInfo: String
});
const To = mongoose.model("To",toSchema);

const invoiceSchema = new mongoose.Schema({
    invId: { type: String },
    date: String,
    issue_date: String,
    from: [{ type: mongoose.Schema.Types.ObjectId, ref: 'From' }],
    to: [{ type: mongoose.Schema.Types.ObjectId, ref: 'To' }],
    description: String,
    pType: String,
    quantity: String,
    amount: String,
    totalAmount: String,
    gst: String
});
const Invoice = mongoose.model("Invoice",invoiceSchema);

app.get("/invoice-generator", (req, res) => {
    Invoice.find()
        .populate('from')
        .populate('to')
        .exec((err, invoices) => {
            if (err) {
                throw err;
            }
            console.log(invoices);
            res.render("invoice-generator", {invoices: invoices});
        });
})

app.post("/invoice-generator", function(req, res) {
    const newFrom = new From({
        title: req.body.from_name,
        email: req.body.from_extra,
        number: req.body.from_address
    });

    newFrom.save((err, savedFrom) => {
        if (err) {
            throw err;
        }
        if (savedFrom) {
            const newTo = new To({
                billing_address: req.body.billing_address,
                billing_name: req.body.billing_name,
                addInfo: req.body.billing_extra
            });
            newTo.save((err, savedTo) => {
                if (err) {
                    throw err;
                }
                if (savedTo) {
                    const newInvoice = new Invoice({
                        invId: req.body.invId,
                        date: req.body.getDate,
                        issue_date: req.body.issue_Date,
                        from: [newFrom._id],
                        to: [newTo._id],
                        description: req.body.item_name,
                        pType: req.body.item_type,
                        quantity: req.body.item_qty,
                        amount: req.body.item_rate,
                        totalAmount: req.body.totalA,
                        gst: req.body.item_gst
                    });
                    newInvoice.save((err, savedInvoice) => {
                        if (err) {
                            throw err;
                        }
                        console.log(savedInvoice);
                        res.send(savedInvoice);
                    })
                }
            });        
        }
    });
});

app.listen(3000, function() {
    console.log("Server started on port 3000");
});

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