Applying condition on localfield in aggregate in mongodb - mongodb

Question

I have two collections user and transaction, transaction has two fields customer and seller , and user has two fields name and email., user collection contains data of customers and sellers
I want output as if the email passed belongs to customer then seller details should come from lookup and if email passed belongs to seller then customer details should come from lookup
What I have done is, I am passing the value email dynamically , email value can belongs to customer or seller , If email matches seller then I want the localfield in lookup as customer and vice versa. Following is what I have tried.
const email = req.email;
transaction.aggregate([{
$match: {
$or: [{
customer: email,
},
{
seller: email,
},
],
},
},
{
$lookup: {
from: "user",
localField: {
$cond: [{
if: {
$eq: ["$customer", email]
},
then: "seller",
else: "customer",
}],
},
foreignField: "email",
as: "user",
},
},
{
$unwind: "$user"
},
]);
But for the above query error comes as below
$lookup argument 'localField' must be a string
I am using nodejs, express and mongoose.

Answer 1

The localField allows only string, you can use lookup with pipeline,
let to define your conditions logic
pipeline to put your matching condition with lookup collection's field
{
$lookup: {
from: "user",
let: {
localField: {
$cond: [{
if: { $eq: ["$customer", email] },
then: "$seller",
else: "$customer",
}]
}
},
pipeline: [
{ $match: { $expr: { $eq: ["$email", "$$localField"] } } }
],
as: "user"
}
}