How to keep/link array of subdocuments in another document

index.js

//index.js
const express = require('express');
const ejs=require('ejs');
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const Coin = require('./models/coins');

mongoose.connect('mongodb://localhost/playground')
.then(()=>console.log('connected'))
.catch(err => console.error('some error',err))

async function createCoin(){
    const Coin = require('./models/coins');
    const coin = new Coin(
        {
        name: 'BTT',
        description: 'Bittorrent coin'
        }
    );

    const result = await coin.save();
    console.log(result);
}

createCoin()

async function createOrder(){
    const {Order} = require('./models/orders');
    const order = new Order(
        {
            purchasePrice: 0.54,
            purchaseCount: 200,
            purchaseAmount: 54000,
            comission:23
    
        }
    );
    
    const result = await order.save();
    addOrder('BTT',result)
    // console.log(result);
    }
    
createOrder()

async function addOrder(coinName,order){
    console.log('coinName', coinName)
    console.log('order', order._doc)
    const coin = await Coin.findOneAndUpdate(
        {name:coinName},
        {$push: {
            order: order._doc
            }}
        );
}

models/coins.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const coinSchema = new mongoose.Schema(
    {
        name: String,
        description: String,
        order: Schema.Types.Mixed        
    }
);

const Coin = mongoose.model('Coin',coinSchema);
module.exports = Coin

models/orders.js

const mongoose = require('mongoose');
const orderSchema = new mongoose.Schema(
    {
        description: String,
        purchasePrice: Number,
        purchaseCount: Number,
        purchaseAmount: Number,
        comission:Number
        
    }
);

const Order = mongoose.model('Order',orderSchema);
module.exports.Order = Order
module.exports.orderSchema = orderSchema

Once the index.js is executed, you can see the data created in mongodb database, which is also created if not there already.