Why my mongoose schema's field which is refrenced to another schema is creating empty array?

Im having 2 different models in my code :

1. User Schema

let userSchema = new Schema({
    _id: Schema.Types.ObjectId,
    name: { type: String, default: '', required: true },
    comments: [{ 
        type: Schema.Types.ObjectId, ref: 'Comment'
    }]
});
const User = model('User', userSchema,'User');

2. Comment Schema

const commentSchema = new Schema({
    author: { type: Schema.Types.ObjectId, ref: 'User'},
    text: { type: String, default: '', required: true },
});

const Comment = model('Comment', commentSchema, 'Comment');

so when I make a new user, it has an empty comments array which is ok

Example user :

{
    "_id": "62da583a27c7bf8a5b3ce118",
    "name": "saleh gholami",
    "comments": []
}

but when I make a new comment, it'll only set the author of the comment object

Example comment :

{
    "author": "62da583a27c7bf8a5b3ce118",
    "text": "Hello World !",
    "_id": "62da583d27c7bf8a5b3ce11b"
}

What I'm expecting to see is that when I save a new comment object, It'd add the id of the comment to a field of user's object which is an array and it's called comments

Example of what I expect :

{
    "_id": "62da583a27c7bf8a5b3ce118",
    "name": "saleh gholami",
    "comments": ["62da583a27c7bf8a5b3ce118"] //array of user's comments ids
}

But I'm still getting the empty array

1. Should I push the new comment ids manually each time ?
2. Or is there any way to automatically sync the array field with the new values?