Budget.js

const mongoose = require('mongoose');
const { Schema } = mongoose;
const TransactionSchema = require('./Transaction');

const budgetSchema = new Schema({
  title: String,
  description: String,
  amount: { type: Number, default: 0 },
  startDate: Date,
  endDate: Date,
  transactions: [TransactionSchema],
  _user: { type: Schema.Types.ObjectId, ref: 'User' },
  dateCreated: Date
})

mongoose.model('budgets', budgetSchema);

budgetRoutes.js

const mongoose = require('mongoose');
const requireLogin = require('../middlewares/requireLogin');
const Budget = mongoose.model('budgets');

module.exports = app => {
  // get budgets
  app.get('/api/budgets', requireLogin, async (req, res) => {
    const budgets = await Budget.find({ _user: req.user.id });

    res.send(budgets);
  });

  app.post('/api/budgets', requireLogin, async (req, res) => {
    const { title, description, amount, startDate, endDate, transactions } = req.body;

    const budget = new Budget({
      title,
      description,
      amount,
      startDate,
      endDate,
      transactions,
      _user: req.user.id,
      dateCreated: Date.now()
    });

    try {
      await budget.save();
      req.user.totalBalance += parseFloat(amount);
      const user = await req.user.save();

      res.send(user);
    } catch (err) {
      res.status(422).send(err)
    }
  });

  app.delete('/api/budgets', requireLogin, async (req, res) => {

  });

};