How to keep id on mongoose findByIdAndUpdate - node.js

Question

I am trying to update a 'Board' model in mongoose using findByIdAndUpdate and the model has an array of 'items' (which are objects) on the model. I probably do not understand mongoose well enough but for some reason each item in the array gets an id generated, along with the Board. This is not a problem, it's quite handy actually, however, after doing a findByIdAndUpdate the id on each item has changed. This was quite surprising to me, I really thought they would stay the same. Could this be caused by updating all items in the array? Maybe mongoose is just throwing out the entire array and creating a new one when updating (maybe someone knows?). Anyways, my question is: Is there a way to update the model without changing these id's. I would really like them to stay consistent. The code I am using for update is
exports.updateBoard = asyncHandler(async (req, res, next) => {
let board = await Board.findById(req.params.id);
if (!board) {
return next(new CustomError(`Board not found with id of ${req.params.id}`, 404));
}
// Authorize user
if (board.user.toString() !== req.user.id) {
return next(new CustomError(`User ${req.user.id} is not authorized to update board ${board._id}`, 401));
}
req.body.lastUpdated = Date.now();
board = await Board.findByIdAndUpdate(req.params.id, req.body, { new: true, runValidators: true })
.select('-__v')
.populate({
path: 'user',
select: 'name avatar',
});
// 200 - success
res.status(200).json({ success: true, data: board });
});
and BoardSchema:
const BoardSchema = new Schema(
{
user: {
type: Schema.Types.ObjectId,
ref: 'User',
required: [true, 'Board must have a user'],
},
name: {
type: String,
required: true,
trim: true,
},
description: {
type: String,
required: false,
trim: true,
},
items: [
{
title: {
type: String,
required: true,
trim: true,
},
description: {
type: String,
required: false,
trim: true,
},
dateCreated: {
type: Date,
default: Date.now,
},
lastUpdated: {
type: Date,
default: Date.now,
},
},
],
columns: [
{
name: {
type: String,
required: true,
},
index: {
type: Number,
required: true,
},
show: {
type: Boolean,
required: true,
},
},
],
dateCreated: {
type: Date,
default: Date.now,
},
lastUpdated: {
type: Date,
default: Date.now,
},
},
{
toJSON: { virtuals: true },
toObject: { virtuals: true },
},
);