I have a collection “products”, another “transactions” and then “users”.
Every product has some transactions and every transaction has a user.
So I want to fetch products having a key-value pair of transactions containing an array based on productId
And then every transaction containing a key-value pair of traveller based on travellerId from users collection.

I have achieved the desired output if there are some transactions but if transacitons are not there. Then instead of having transactions: [], it sends me back transactions: [{}]

I am running this query:

const tripProducts = await db.collection('products')
                .aggregate([
                    { $match: { tripId: new ObjectId(tripId ?? '') } },
                    { $lookup: { from: 'transactions', localField: '_id', foreignField: 'productId', as: 'transactions' } },
                    { $unwind: { path: '$transactions', preserveNullAndEmptyArrays: true } }, // Unwind transactions array
                    { $lookup: { from: 'users', localField: 'transactions.travellerId', foreignField: '_id', as: 'traveller' } },
                    { $unwind: { path: '$traveller', preserveNullAndEmptyArrays: true } }, // Unwind traveller array
                    { $addFields: { 'transactions.traveller': '$traveller' } },
                    {
                        $group: {
                            _id: '$_id',
                            // Include all fields from products using $first or similar operators
                            tripId: { $first: '$tripId' },
                            name: { $first: '$name' },
                            category: { $first: '$category' },
                            productPrice: { $first: '$productPrice' },
                            currency: { $first: '$currency' },
                            depositForBusiness: { $first: '$depositForBusiness' },
                            depositDate: { $first: '$depositDate' },
                            finalDate: { $first: '$finalDate' },
                            minTravellers: { $first: '$minTravellers' },
                            maxTravellers: { $first: '$maxTravellers' },
                            amount: { $first: '$amount' },
                            depositAmount: { $first: '$depositAmount' },
                            platformFeeForDeposit: { $first: '$platformFeeForDeposit' },
                            platformFee: { $first: '$platformFee' },
                            userId: { $first: '$userId' },
                            transactions: { $push: '$transactions' } // Push the transactions with the traveller details
                        }
                    }
                ])
                .toArray();

And my output is:

[
    {
        "_id": "66d62cb0caeb8c9e204a10b5",
        "tripId": "66d62bc5caeb8c9e204a10b3",
        "name": "Next Year",
        "category": "🏨 Stay",
        "productPrice": 10000,
        "currency": "usd",
        "depositForBusiness": 1000,
        "depositDate": "2024-09-04T19:00:00.000Z",
        "finalDate": "2025-04-09T19:00:00.000Z",
        "minTravellers": 1,
        "maxTravellers": 2,
        "amount": 10975.6,
        "depositAmount": 1098.1,
        "platformFeeForDeposit": 98.1,
        "platformFee": 975.6000000000004,
        "userId": "66bb9220366e6c4942f4943e",
        "transactions": [
            {}
        ]
    },
    {
        "_id": "66d73682b976159272117780",
        "tripId": "66d62bc5caeb8c9e204a10b3",
        "name": "Tomorrow",
        "category": "🏨 Stay",
        "productPrice": 1000,
        "currency": "dkk",
        "depositForBusiness": 100,
        "depositDate": "2024-09-03T19:00:00.000Z",
        "finalDate": "2024-09-03T19:00:00.000Z",
        "minTravellers": 1,
        "maxTravellers": 2,
        "amount": 1098.1,
        "depositAmount": 110.35,
        "platformFeeForDeposit": 10.35,
        "platformFee": 98.09999999999991,
        "userId": "66bb9220366e6c4942f4943e",
        "transactions": [
            {}
        ]
    }
]