title: Cannot find module 'mongoose-unique-validator'
body: <p>I'm trying to create a todo list App with passport authentication, but I get this error when I run node app:</p>
  
  <pre><code>module.js:549
      throw err;
      ^
  
  Error: Cannot find module 'mongoose-unique-validator'
      at Function.Module._resolveFilename (module.js:547:15)
      at Function.Module._load (module.js:474:25)
      at Module.require (module.js:596:17)
      at require (internal/module.js:11:18)
      at Object.&lt;anonymous&gt; (/home/walid/Todo list API/models/user.js:2:23)
      at Module._compile (module.js:652:30)
      at Object.Module._extensions..js (module.js:663:10)
      at Module.load (module.js:565:32)
      at tryModuleLoad (module.js:505:12)
      at Function.Module._load (module.js:497:3)
      at Module.require (module.js:596:17)
      at require (internal/module.js:11:18)
      at Object.&lt;anonymous&gt; (/home/walid/Todo list API/app.js:7:12)
      at Module._compile (module.js:652:30)
      at Object.Module._extensions..js (module.js:663:10)
      at Module.load (module.js:565:32)
  </code></pre>
  
  <p>This is my App file, and the error says that it can't connect to mongoose:</p>
  
  <pre><code>const express = require('express');
  const path = require('path');
  const mongoose = require('mongoose');
  const bodyParser = require('body-parser'); 
  const expressValidator = require('express-validator');
  const flash = require('connect-flash');
  const session = require('express-session');
  const config = require('./config/database');
  const passport = require('passport');
  
  mongoose.connect(config.database);
  let db = mongoose.connection;
  
  // Check connection
  db.once('open', function () {
      console.log('Connected to mongoDB');
  });
  </code></pre>
  
  <p>Can anyone please help me?</p>