variable undefined after findOne operation mongodb - mongodb

Question

I am trying to make an API that makes use of 2 databases to generate a fine. Here is the code:
router.get("/generateFine/:bookingID/:currDate", function (req, res, next) {
var currDate,
returnDate,
fine,
totalFine = 0;
Booking.findOne({ _id: req.params.bookingID }).then(function (booking) {
Car.findOne({ _id: booking.carID }).then(function (car) {
currDate = Date.parse(req.params.currDate) / 1000 / 3600 / 24;
returnDate = Date.parse(booking.bookingDates[1]) / 1000 / 3600 / 24;
fine = car.fine;
if (currDate > returnDate) {
totalFine = fine * (currDate - returnDate);
}
console.log(totalFine);
// res.send(totalFine);
});
console.log("totalFine is " + totalFine);
// res.send(totalFine);
});
});
Here are the two Schemas used in the code:
Booking Schema:
{
"_id" : ObjectId("621bf46602edf12942f0d5c9"),
"carID" : "621b87af70c150da70b1dabf",
"bookingDates" : [
"2022-03-05",
"2022-03-06"
],
}
Car Schema:
{
"_id" : ObjectId("621b87af70c150da70b1dabf"),
"name" : "Toyota",
"rate" : 60,
"fine" : 10,
"datesBooked" : [
{
"from" : "2022-03-05",
"to" : "2022-03-06"
},
{
"from" : "2022-03-07",
"to" : "2022-03-08"
},
{
"from" : "2022-03-09",
"to" : "2022-03-10"
}
],
"__v" : 0
}
I want to return the generated fine to the user. When I am trying to send the result, it throwing an error. The first console log prints the correct result, but the second console log prints 0. Also, how can I send the result without getting an error.
Thanks already!

Answer 1

You could use $lookup aggregation pipeline stage to include the car document that matches on the carID field, create additional computed fields that will aid you in getting the total fine whilst using the necessary aggregation operators.
Essentially you would need to run an aggregate pipeline that follows:
const mongoose = require('mongoose');
router.get('/generateFine/:bookingID/:currDate', async function (req, res, next) {
const currDate = new Date(req.params.currDate);
const [{ totalFine }] = await Booking.aggregate([
{ $match: { _id: mongoose.Types.ObjectId(req.params.bookingID) }},
{ $lookup: {
from: 'cars', // or from: Car.collection.name
let: { carId: { $toObjectId: '$carID' } }, // convert the carID string field to ObjectId for the match to work correctly
pipeline: [
{ $match: {
$expr: { $eq: [ '$_id', '$$carId' ] }
} }
],
as: 'car'
} },
{ $addFields: {
car: { $arrayElemAt: ['$car', 0 ] }, // get the car document from the array returned above
returnDate: {
$toDate: { $arrayElemAt: ['$bookingDates', 1 ]}
}
} },
// compute the overdue days
{ $addFields: {
overdueDays: {
$trunc: {
$ceil: {
$abs: {
$sum: {
$divide: [
{ $subtract: [currDate, '$returnDate'] },
60 * 1000 * 60 * 24
]
}
}
}
}
}
} },
{ $project: { // project a new field
totalFine: {
$cond: [
{ $gt: [currDate, '$returnDate'] }, // IF current date is greater than return date
{ $multiply: ['$car.fine', '$overdueDays'] }, // THEN multiply car fine with the overdue days
0 // ELSE total fine is 0
]
}
} }
]).exec();
console.log("totalFine is " + totalFine);
// res.send(totalFine);
});