Mongodb – MongoMemoryServer or mongoose in Jest not providing Schema _id

I am running node version 16.15.0 and have the package.json dependencies:

"jest": "^28.1.0",
"mongodb-memory-server": "^8.6.0",
"mongoose": "^6.3.5",

I have a mongoose.Schema set up in a User.js module:

import mongoose from 'mongoose'
import validator from 'validator'
import bcrypt from 'bcryptjs'
import jwt from 'jsonwebtoken'

const UserSchema = new mongoose.Schema({
  name: {
    type: String,
    required: [true, 'Please provide name'],
    minlength: 3,
    maxlength: 20,
    trim: true,
  },
  email: {
    type: String,
    required: [true, 'Please provide email'],
    validate: {
      validator: validator.isEmail,
      message: 'Please provide a valid email',
    },
    unique: true,
  },
  password: {
    type: String,
    required: [true, 'Please provide password'],
    minlength: 6,
    select: false,
  },
  role: {
    type: String,
    trim: true,
    maxlength: 20,
    default: 'PLAYER',
  },
})

UserSchema.pre('save', async function () {
  // console.log(this.modifiedPaths())
  if (!this.isModified('password')) return
  const salt = await bcrypt.genSalt(10)
  this.password = await bcrypt.hash(this.password, salt)
})

UserSchema.methods.createJWT = function () {
  return jwt.sign({ userId: this._id }, process.env.JWT_SECRET, {
    expiresIn: process.env.JWT_LIFETIME,
  })
}

UserSchema.methods.comparePassword = async function (candidatePassword) {
  const isMatch = await bcrypt.compare(candidatePassword, this.password)
  return isMatch
}

export default mongoose.model('User', UserSchema)

(note the UserSchema.methods.createJWT = function, because this is in the test below)

and finally a simple Jest test (I am only just starting out with Jest):

import mongoose from 'mongoose'
import dotenv from 'dotenv'
import { MongoMemoryServer } from 'mongodb-memory-server'
import User from '../../models/User.js'

describe('User Schema suite', () => {
  dotenv.config()

  const env = process.env

  var con, mongoServer

  beforeAll(async () => {
    mongoServer = await MongoMemoryServer.create()
    con = await mongoose.connect(mongoServer.getUri(), {})

    jest.resetModules()
    process.env = { ...env }
  })

  afterAll(async () => {
    if (con) {
      con.disconnect()
    }
    if (mongoServer) {
      await mongoServer.stop()
    }

    process.env = env
  })

  test('should read the environment vars', () => {
    expect(process.env.JWT_SECRET).toBeTruthy()
    expect(process.env.JWT_SECRET).toEqual('?E(H+MbQeThWmYq3t6w9z$C&F)J@NcRf')

    expect(process.env.JWT_LIFETIME).toBeTruthy()
    expect(process.env.JWT_LIFETIME).toEqual('1d')
  })

  test('should create and sign a good token', async () => {
    const user = await User.create({
      name: 'Mike',
      email: '[email protected]',
      password: 'secret',
    })

    expect(user.createJWT()).toBeTruthy()
  })
})

BTW: I also tried to add the _id in manually with this User.create expression

    const user = await User.create({
      _id: '62991d39873ec2778e34f114',
      name: 'Mike',
      email: '[email protected]',
      password: 'secret',
    })

But it made no difference.

The first test passes, however the second one fails with the following error:

mike@mike verser % npm test

> [email protected] test
> jest --testEnvironment=node --runInBand ./tests

 FAIL  tests/models/User.test.js
  ● User Schema suite › should create and sign a good token

    TypeError: Cannot read properties of null (reading 'ObjectId')

      at Object.<anonymous> (node_modules/mongoose/lib/types/objectid.js:13:44)
      at Object.<anonymous> (node_modules/mongoose/lib/utils.js:9:18)


Test Suites: 1 failed, 1 skipped, 1 of 2 total
Tests:       1 failed, 1 skipped, 1 passed, 3 total
Snapshots:   0 total
Time:        1.127 s, estimated 2 s
Ran all test suites matching /./tests/i.

The code in User.js works in production where I am using Mongo Community v5.0.7 (in a docker container).

So why can’t I access the _id value when I am using MongoMemoryServer instead? Is there something that I need to set? Or is there something else that I’m doing wrong?