_id is missing in subDoc

[abarriel]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.6.3

Node.js version

16

MongoDB server version

4.9.1

Description

Hello, _id is missing from subDoc when saving. It shouldn't, _id should be set in array. You can run the script to see Animal Elephant is missing _id property.

Steps to Reproduce

const mongoose = require("mongoose");

const Schema = mongoose.Schema;

const animalSchema = new Schema(
  {
    title: String,
  },
  { _id: true },
);

const animalsSchema = new Schema({
  species: [animalSchema],
  totalAnimals: Number,
});

const Userschema = new Schema({
  animals: animalsSchema,
});

const UserModel = mongoose.model("UserAnimal", Userschema);

(async () => {
  await mongoose.connect(`mongodb://localhost:27017`, {
    dbName: "userSchema",
  });

 // init one doc
  const doc = new UserModel();
  doc.animals = { totalAnimals: 1 };
  doc.animals.species = [{ title: "Lion" }];
  await doc.save();

  // once created we fetch it again
  const user = await UserModel.findById(doc._id);
  console.log({ user });
  
  // add new animal
  user.animals.species.push({ title: "Elephant" });
  await user.save();
  console.log("Elephant is missing _id !", user.animals.species);

  await mongoose.disconnect();
})();

Expected Behavior

No response

[hasezoey]

can reproduce

Code used

Ignore the ts errors when using with typescript

// NodeJS: 18.8.0
// MongoDB: 5.0 (Docker)
// Typescript 4.8.4
import * as mongoose from 'mongoose'; // mongoose@6.6.3

const Schema = mongoose.Schema;

const animalSchema = new Schema(
  {
    title: String,
  },
  { _id: true }
);

const animalsSchema = new Schema({
  species: [animalSchema],
  totalAnimals: Number,
});

const Userschema = new Schema({
  animals: animalsSchema,
});

const UserModel = mongoose.model('UserAnimal', Userschema);

(async () => {
  await mongoose.connect(`mongodb://localhost:27017/`, {
    dbName: 'verifyMASTER',
  });

  // init one doc
  const doc = new UserModel();
  doc.animals = { totalAnimals: 1 };
  doc.animals.species = [{ title: 'Lion' }];
  await doc.save();

  // once created we fetch it again
  const user = await UserModel.findById(doc._id);
  console.log(user);

  // add new animal
  user.animals.species.push({ title: 'Elephant' });
  await user.save();
  console.log('Elephant is missing _id !', user.animals.species);

  await mongoose.disconnect();
})();

output:

{
  _id: new ObjectId("633c4d1f2441695e1b7dc3d3"),
  animals: {
    totalAnimals: 1,
    _id: new ObjectId("633c4d1f2441695e1b7dc3d4"),
    species: [ [Object] ]
  },
  __v: 0
}
Elephant is missing _id ! [
  { title: 'Lion', _id: new ObjectId("633c4d1f2441695e1b7dc3d5") },
  { title: 'Elephant' }
]