Value pushed to array not persisted

[jyrkive]

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

Bug

What is the current behavior?

If you push a value to an array, populate the said array, push nothing to the said array, and finally save the document, the originally pushed value is not persisted to the database.

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

const mongoose = require("mongoose");
mongoose.connect("mongodb://localhost:27017/array-empty-push", {useNewUrlParser: true});

console.log(`Mongoose ${mongoose.version}`);

const Car = mongoose.model("Car", new mongoose.Schema({
  model: Number
}));

const Player = mongoose.model("Player", new mongoose.Schema({
  cars: [{type: mongoose.Schema.Types.ObjectId, ref: "Car"}]
}));

async function test() {
  await Player.deleteMany({});
  await Car.deleteMany({});

  let car = await Car.create({model: 0});
  await Player.create({cars: [car._id]});
  let player = await Player.findOne({});
  await player.populate("cars").execPopulate();

  console.log(player.cars.length);

  car = await Car.create({model: 1});
  player.cars.push(car);
  await player.populate("cars").execPopulate();

  car = await Car.create({model: 2});
  player.cars.push(car);

  console.log(player.cars.length);

  await player.save();
  player = await Player.findOne({});

  console.log(player.cars.length);

  mongoose.disconnect();
}

test();

What is the expected behavior?

The three numbers this script should print are 1, 3, 3. The actual values it prints are 1, 3, 2.

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

• Node.js 12.4.0
• Mongoose 5.5.14
• MongoDB driver 3.2.7
• MongoDB 3.4.6