index.js

const express = require('express')
const mongoose = require('mongoose')
const User = require('./User')
const Joi = require('@hapi/joi')

const app = express()

const dbOptions = { useNewUrlParser: true, useUnifiedTopology: true }

mongoose.connect('mongodb://localhost:27017/validation-testing', dbOptions, (err) => {
  if (err) {
    console.log('db down ❌')
  } else {
    console.log('db up ✅')
  }
})

app.use(express.json())

const schema = Joi.object().keys({
  username: Joi.string(),
  password: Joi.string()
})

const validate = (req, res, next) => {
  const result = schema.validate(req.body)
  if (result.error) {
    res.status(500).send(result.error.details[0].message)
  } else {
    next()
  }
}

app.post('/create', validate, async (req, res) => {
  try {
    const user = new User(req.body)
    res.send(await user.save())
  } catch(err) {
    res.status(500).send(err)
  }
})

app.listen(5000, () => console.log('listening on port 5000'))

package.json

{
  "name": "mongoose-validation",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "nodemon index.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@hapi/joi": "^16.1.8",
    "celebrate": "^10.1.0",
    "express": "^4.17.1",
    "mongoose": "^5.7.12"
  }
}

User.js

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const myUniqueValidator = async (username) => {
  const doc = await User.findOne({username: username})
  if (doc) {
    return false
  } else {
    return true
  }
} 

const userSchema = new Schema({
  username: {
    type: String,
    required: true,
    validate: [myUniqueValidator, "username must be unique"]
  },
  password: String
})

const User = mongoose.model('User', userSchema)

module.exports = User