How to update only specific field in object within an array Mongodb - node.js

Question

Expense Tracker application : Nodejs, Mongodb
Trying to Create a function that will update only the passed fields from request inside an array of objects
Database Schema
const updateExpense = async (req, res) => {
try {
let db = mongo.getDb()
let { macro, micro, amount, note } = req.body;
let { username, id } = req.query
let expense = await db.collection("Expense").updateOne({ username: username, "expenses.expense_id": ObjectId(id) }, { $set: {
"expenses.$.macro": macro,
"expenses.$.micro": micro,
"expenses.$.amount": amount,
"expenses.$.note": note }
});
res.status(200).json({
message: "Expense Updated",
expense: expense
});
} catch (err) {
res.status(500).json({
message: err.message
});
}
}
The above function is replacing all other fields with null
If the user is passing only the micro field, then the other fields should remain the same and only the micro field should change and other fields should not change.
Need A MongoDB Query which will only change what is required based on the data passed in req

Answer 1

I think you must first fetch from the database with findOne then update that fields set in req.body, something like this:
const updateExpense = async (req, res) => {
try {
let db = mongo.getDb()
let { macro, micro, amount, note } = req.body;
let { username, id } = req.query
let expense = await db.collection("Expense").findOne({ username: username });
let special_ex = expense.expenses.find(ex => ex.expense_id === ObjectId(id);
special_ex.macro = macro ? macro : special_ex.macro;
special_ex.micro = micro ? micro : special_ex.micro;
/*
and so on ...
*/
await expense.update();
res.status(200).json({
message: "Expense Updated",
expense: expense
});
} catch (err) {
res.status(500).json({
message: err.message
});
}
}