update to subdocument didn't update parent document code example

Example 1: mongoose update subdocument by id

//this method for add data to subdocument
BlogPost.findById(req.params.postId, function (err, post) {
    var subDoc = post.comments.id(req.params.commentId);
    subDoc = req.body;
    post.save(function (err) {
        if (err) return res.status(500).send(err);
        res.send(post);
    });
});

// alternative second method you can use this
BlogPost.findOneAndUpdate({_id: req.params.postId}, {$push:{ subDoc: req.body }}, (err, doc) => {
  	// do something here
});

Example 2: mongoose update subdocument by id

const mongoose = require('mongoose');
const bcrypt = require('bcryptjs');
const saltRounds = 10;
const Role = require('../config/roles')
const subscription = require('./subscriptionModel')

const Schema = mongoose.Schema;

const userModel = new Schema({
    created: { type: Date, default: Date.now },
    updated: { type: Date, default: Date.now},
    fullname: {
        type: String,
        required: true
    },
    email: {
        type: String,
        required: true,
        unique: true
    },
    birthDate: {
        type: String,
        required: true
    },
    password: {
        type: String,
        required: true
    },
    profileImage: {
        type: String
    },
    role: {
        type: String,
        default: Role.user
    },
    subscriptions: [subscription.modelName]
})

// hash user password before saving into database
userModel.pre('save', async function save(next) {
    if (!this.isModified('password')) return next();
    try {
      const salt = await bcrypt.genSalt(saltRounds);
      this.password = await bcrypt.hash(this.password, salt);
      return next();
    } catch (err) {
      return next(err);
    }
});
  
// Compare passwords when user tries to log in.
userModel.methods.comparePassword = function(candidatePassword, cb) {
    bcrypt.compare(candidatePassword, this.password, function(err, isMatch) {
        if (err) return cb(err);
        cb(null, isMatch);
    });
};

module.exports = mongoose.model('User', userModel)