index.js

const express = require('express')
const mongoose = require('mongoose')
const User = require('./models/User')
const { celebrate, Joi, errors } = require('celebrate');

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 validate = celebrate({
  body: Joi.object().keys({
    username: Joi.string(),
    password: Joi.string()
  })
})

app.use(validate)
app.use(errors())

app.post('/create', 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": {
    "celebrate": "^10.1.0",
    "express": "^4.17.1",
    "mongoose": "^5.7.12"
  }
}

User.js

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

const myValidator = (username) => {
  return username === "harrison"
} 

const userSchema = new Schema({
  username: {
    type: String,
    validate: [myValidator, "name must be harrison error"]
  },
  password: String
})

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

module.exports = User