How to decrease coins of users in MongoDB?

Question

I am a building a web-app where people have 10 coins by default, and when they click a buy button, it reduces the number of coins by 10 with each click.

The schema for users is like this:

    const mongoose = require("mongoose");
    const UserSchema = new mongoose.Schema(
      {
        username: {
          type: String,
          require: true,
          min: 3,
          max: 20,
          unique: true,
        },
        email: {
          type: String,
          required: true,
          max: 50,
          unique: true,
        },
        password: {
          type: String,
          required: true,
          min: 6,
        },
        isAdmin: {
          type: Boolean,
          default: false,
        },
        coins: {
          type: Number,
          default: 10,
        },
      },
      { timestamps: true } 
    );
    
    module.exports=mongoose.model("User", UserSchema);

How do I write a code to use a button to reduce the number of coins?

Answer 1

• Create /update-user-balance endpoint with handler for PUT request
• Pass user_id and amount in the request body
• If user_id or amount are not passed return error
• findOneAndUpdate with $inc to update balance of the user with user_id by negative amount

const express = require('express');
const Users = require('../models/user');

const router = express.Router();

router.put('/api/v1/update-user-balance', async (req, res, next) => {
  try {
    const { user_id, amount } = req.body;

    if (!user_id || !amount){
      console.log('ERROR: "user_id" and "amount" data are required.';
      return res.status(400).json({ success: false });
    }
    
    await Users.findOneAndUpdate(
      { _id: user_id },
      { $inc: { coins: amount * -1 },
    });
    
    console.log('User balance successfully updated')
    return res.status(200).json({ success: true });
  } catch (error) {
    console.log('ERROR: ', error);
    return res.status(400).json({ success: false });
  }
});

module.exports = router;

Answer 2

https://mongoosejs.com/docs/documents.html

Refer the above documentation.

You can update documents using queries in mongoose.

Ex:

await model.updateMany({}, { $set: { coins: 10 } });

You can set the value of coins parameter in query and update.

Before updating the document, you need to take the document using model.findOne fuction and get the available coins of the user.

After that you need to do -10 calculation and update the document.