节点/Mongoose错误: ValidationError

Question

这是我的server.js。当我运行node server.js然后使用PostMan来post json时，它给出了以下错误。

var express = require('express')
var bodyParser = require('body-parser')
var app = express()

app.use(bodyParser.json())

app.get('/api/posts', function(req, res) {
  res.json([
    {
      username: '@rodandrew95',
      body: 'node rocks!'
    }
  ])
})

app.listen(3000, function() {
  console.log('Server listening on', 3000)
})


var Post = require('./models/post')
app.post('/api/posts', function(req, res, next) {
  // console.log('post received')
  // console.log(req.body.username)
  // console.log(req.body.body)
  // res.sendStatus(201)
  var post = new Post({
    username: req.body.username,
    body: req.body.body
  });
  post.save(function (err, post) {
    if (err) { return next(err) }
    res.sendStatus(201).json(post)
  })
})

错误：

(node:6863) DeprecationWarning: Mongoose: mpromise (mongoose's default promise library) is deprecated, plug in your own promise library instead: http://mongoosejs.com/docs/promises.html
ValidationError: Post validation failed
    at MongooseError.ValidationError (/Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/error/validation.js:23:11)
    at model.Document.invalidate (/Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/document.js:1486:32)
    at /Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/document.js:1362:17
    at validate (/Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/schematype.js:705:7)
    at /Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/schematype.js:742:9
    at Array.forEach (native)
    at SchemaString.SchemaType.doValidate (/Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/schematype.js:710:19)
    at /Users/andrewrodrigues/Desktop/write_modern/ch_1/node_modules/mongoose/lib/document.js:1360:9
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)

我试图通过“用MEAN stack编写现代Web应用程序”来学习MEAN堆栈，但我总是遇到问题，即使我完全遵循代码和说明。有没有人可以帮助理解这个错误，并可能推荐一些好的资源来学习mean堆栈？