why do i keep getting OverwriteModelError? - node.js

Question

I get the following error when I start the server:
throw new _mongoose.Error.OverwriteModelError(name);
^ OverwriteModelError: Cannot overwrite User model once compiled.
at new OverwriteModelError (/home/pranav/exercise-tracker/mern-exercise-tracker/backend/node_modules/mongoose/lib/error/overwriteModel.js:20:11)
my user.model.js file
const mongoose = require("mongoose");
const Schema = mongoose.Schema;
const userSchema = new Schema(
{
username: {
type: String,
required: true,
unique: true,
trim: true,
minlength: 3
}
},
{
timestamps: true
}
);
const User = mongoose.model("User", userSchema);
module.exports = User;
My users.js
const router = require("express").Router();
let User = require("../models/user.model");
router.route("/").get((req, res) => {
User.find()
.then(users => res.json(users))
.catch(err => res.status(400).json("Error: " + err));
});
router.route("/add").post((req, res) => {
const username = req.body.username;
const newUser = new User({ username });
newUser
.save()
.then(() => res.json("User added!"))
.catch(err => res.status(400).json("Error: " + err));
});
module.exports = router;
server.js
const express = require('express');
const cors = require('cors');
const mongoose = require('mongoose');
require('dotenv').config();
const app = express();
const port = process.env.PORT || 5000;
app.use(cors);
app.use(express.json());
const uri = process.env.ATLAS_URI;
mongoose.connect(uri, { useNewUrlParser: true, useCreateIndex: true, useUnifiedTopology: true });
const connection = mongoose.connection;
connection.once('open', () => {
console.log("MongoDB connection established");
})
const exercisesRouter = require('./routes/exercises');
const usersRouter = require('./routes/users');
app.use('/exercises', exercisesRouter);
app.use('/users', usersRouter);
app.listen(port, () => {
console.log('Server is running on port : ' + port);
})

Answer 1

Replace
const User = mongoose.model("User", userSchema);
with
const User = mongoose.models['User'] || mongoose.model("User", userSchema);
When your code is running user.model.js is probably being called again, thus calling the initial definition.
Note the models being used in the OR. That method returns an object containing all defined models. It can also be written as mongoose.models.User.