MongoDB get $lookup leftovers - mongodb

Question

I'm trying to implement a query that will merge two collections into one and this can be done using $lookup. What I'm trying to achieve is that if the localField doesn't match foreignField I wanted that record to be included as well. Is this possible or am I missing something? Below is what I've tried so far which is a basic $lookup aggregation:
const billPayments = await BillPayment.aggregate([
{
$match: {
isActive: true,
},
},
{
$lookup: {
from: PackageList.collection.name,
localField: "serviceNumber",
foreignField: "serviceNumber",
as: "package",
},
},
{ $unwind: "$package" },
{
$project: {
serviceNumber: 1,
amount: 1,
isCaution: 1,
paymentDate: 1,
remark: 1,
package: "$package",
id: "$_id",
},
},
]);
res.json(billPayments);
The current result of billPayments is only for those BillPayment data which serviceNumber exists in both collections. But I wanted those records which don't exist in PackageList to be included as well. How can I achieve that?
I'm pretty sure I'm missing something here. Any help is appreciated.
#Edit for expected output
"billPayments": [
{
"amount": 299.25,
"id": "604f5bbd6bf57e1f9c58203f",
"isActive": true,
"isCaution": false,
"packageService": {
"_id": "5fc4a5e15ce5bf227451f4b8",
"isActive": true,
"serviceType": "Post_Paid",
"moneyLimit": 3000
},
"employee": {
// Employee data from Employee collection to be included
}
"paymentDate": "2021-03-14T21:00:00.000Z",
"remark": "",
"serviceNumber": "0935998681"
},
{
"amount": 299.25,
"id": "604f5bbd6bf57e1f9c58203f",
"isActive": true,
"packageService": {
// If there is no match for serviceNumber
},
"paymentDate": "2021-03-14T21:00:00.000Z",
"remark": "",
"serviceNumber": "0735448621"
}
]
Edit for additional $lookup query:
I have got a second level $lookup to get additional information from another collection. Which is like below:
{
$lookup: {
from: PackageList.collection.name,
localField: "serviceNumber",
foreignField: "serviceNumber",
as: "package",
},
},
{ $unwind: "$package" },
{
$lookup: {
from: Employee.collection.name,
localField: "package.employee",
foreignField: "_id",
as: "employee",
},
},
As #DheemanthBhat suggested in his answer, I've tried to remove unwind from Package and can easily get an empty list. But if I tried to $lookup on Package to get the employee information, it throws MongoError since it cannot lookup on an empty object.
Is there any method to skip those empty packages from a second level $lookup? Or how can I use $ifNull in this case to skip $unwind from empty results?

Answer 1

So when you perform $lookup without $unwind all the details from BillPayment collection matched or unmatched with the PackageList collection will be available. If you $unwind then the documents with empty packageService will be removed. Based on your expected output If you want an empty packageService object then check the below query.
Note: Please verify the collection/model name and field names before using the below code.
const billPayments = await BillPayment.aggregate([
{
$match: { isActive: true }
},
{
$lookup: {
from: "PackageList",
localField: "serviceNumber",
foreignField: "serviceNumber",
as: "packageService"
}
},
{
$project: {
serviceNumber: 1,
amount: 1,
isCaution: 1,
paymentDate: 1,
remark: 1,
package: "$package",
id: "$_id",
packageService: {
$ifNull: [{ $arrayElemAt: ["$packageService", 0] }, {}]
}
}
}
]);
Output:
/* 1 createdAt:3/15/2021, 6:36:05 PM*/
{
"_id" : ObjectId("604f5bbd6bf57e1f9c58203f"),
"isCaution" : false,
"amount" : 299.25,
"paymentDate" : ISODate("2021-03-15T02:30:00.000+05:30"),
"serviceNumber" : "0935998681",
"id" : ObjectId("604f5bbd6bf57e1f9c58203f"),
"packageService" : {
"_id" : ObjectId("5fc4a5e15ce5bf227451f4b8"),
"serviceNumber" : "0935998681",
"isActive" : true,
"serviceType" : "Post_Paid",
"moneyLimit" : 3000
}
},
/* 2 createdAt:3/15/2021, 6:36:05 PM*/
{
"_id" : ObjectId("604f5bbd6bf57e1f9c582040"),
"isCaution" : false,
"amount" : 300,
"paymentDate" : ISODate("2021-03-16T02:30:00.000+05:30"),
"serviceNumber" : "0735448621",
"id" : ObjectId("604f5bbd6bf57e1f9c582040"),
"packageService" : {
}
}
Test data:
BillPayment collection
{
"_id" : ObjectId("604f5bbd6bf57e1f9c58203f"),
"isActive" : true,
"isCaution" : false,
"amount" : 299.25,
"paymentDate" : ISODate("2021-03-15T02:30:00.000+05:30"),
"serviceNumber" : "0935998681"
},
{
"_id" : ObjectId("604f5bbd6bf57e1f9c582040"),
"isActive" : true,
"isCaution" : false,
"amount" : 300,
"paymentDate" : ISODate("2021-03-16T02:30:00.000+05:30"),
"serviceNumber" : "0735448621"
}
PackageList collection
{
"_id" : ObjectId("5fc4a5e15ce5bf227451f4b8"),
"serviceNumber" : "0935998681",
"isActive" : true,
"serviceType" : "Post_Paid",
"moneyLimit" : 3000
}