Bug: Mongoose pre validate sub doc middleware Unhandled Rejection

bug.js

let mongoose = require('mongoose')
let Promise = require('bluebird')
mongoose.Promise = Promise

mongoose.connect("mongodb://localhost/bug")
let db = mongoose.connection
db.on('error', (err) => {
	return console.error('Connection error:', err)
})


// child schema
let childSchema = new mongoose.Schema({
	someValue: String
})



// COMMENT OUT MIDDLEWARE TO SEE THE BEHAVIOR OF OTHERS
childSchema.pre('validate', (next) => {
	next(new Error('Error: child pre validate'))
})

childSchema.pre('save', (next) => {
	next(new Error('Error: child pre save'))
})



let Child = mongoose.model('Child', childSchema)


// parent schema
let parentSchema = new mongoose.Schema({
	// NOTE: child model must be created already or its middleware will not trigger at all
	child: childSchema // Child.schema
})



// COMMENT OUT MIDDLEWARE TO SEE THE BEHAVIOR OF OTHERS
parentSchema.pre('validate', (next) => {
	next(new Error('Error: parent pre validate'))
})

parentSchema.pre('save', (next) => {
	next(new Error('Error: parent pre save'))
})



let Parent = mongoose.model('Parent', parentSchema)


// trigger run process where events trigger
let test = new Parent()
test.child = {}
test.child.someValue = "test"

console.log(test)

test.save()
	.then((test) => {
		console.log("Successfully saved!")
	})
	.catch((err) => {
		console.log(err.message)
	})

package.json

{
  "name": "bug-report2",
  "version": "1.0.0",
  "description": "",
  "main": "bug.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "bluebird": "^3.5.0",
    "mongoose": "^4.9.7"
  }
}