bulkSave() throws an error when an unchanged document is passed

[lucashpmelo]

Do you want to request a feature or report a bug?

A bug

What is the current behavior?

When an unmodified object is passed to bulkSave() an error is raised:
TypeError: Cannot read property '0' of undefined

If the current behavior is a bug, please provide the steps to reproduce.

const mongoose = require('mongoose')
const { Schema } = mongoose

const testSchema = new Schema({
  name: {
    type: String,
    required: true,
  },
  date: {
    type: Date,
    required: true,
  },
  value: {
    type: Number,
    required: true,
    default: 0,
  },
})

new mongoose.model('Test', testSchema)
const Test = mongoose.model('Test')

async function bulkSave(data) {
  const bulk = []

  for (const i in data) {
    const { name, date, value } = data[i]

    let test = await Test.findOne({ name: name, date: date })

    if (!test) {
      test = new Test(data[i])
    }

    test.value = value

    bulk.push(test)
  }

  return await Test.bulkSave(bulk)
}

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true,
  })

  await mongoose.connection.dropDatabase()

  const data1 = [
    { name: 'Test 1', date: '2021-08-01T00:00:00.000Z', value: 10 },
    { name: 'Test 1', date: '2021-08-02T00:00:00.000Z', value: 20 },
    { name: 'Test 2', date: '2021-08-01T00:00:00.000Z', value: 15 },
  ]

  await bulkSave(data1)

  const data2 = [
    { name: 'Test 1', date: '2021-08-01T00:00:00.000Z', value: 15 },
    { name: 'Test 1', date: '2021-08-02T00:00:00.000Z', value: 20 }, // <-- ERROR
    { name: 'Test 2', date: '2021-08-02T00:00:00.000Z', value: 25 },
  ]

  await bulkSave(data2)

  console.log('Done')
}

run().catch((err) => console.log(err))

What is the expected behavior?

bulkSave() handle whether the document has changes or not.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node v13.11.0
Mongoose v5.13.2
MongoDB 4.2.5