Aggregate multiple lookups return no data - mongodb

Question

I have documents like this in DB. And I need to grab the data of each item based on the itemType from their own collection.
{ listId: 2, itemType: 'book', itemId: 5364 },
{ listId: 2, itemType: 'car', itemId: 354 },
{ listId: 2, itemType: 'laptop', itemId: 228 }
Based on MongoDB docs and some search, I figured out that I need to use let and $expr in lookup, to make some condition.
ListItemsModel.aggregate([
{ $match: { listId: 2 } },
{ $lookup:
{
from: 'books',
localField: 'itemId',
foreignField: '_id',
let: { "itemType": "$itemType" },
pipeline: [
{ $project: { _id: 1, title: 1 }},
{ $match: { $expr: { $eq: ["$$itemType", "book"] } }}
],
as: 'data'
}
},
{ $lookup:
{
from: 'cars',
localField: 'itemId',
foreignField: '_id',
let: { "itemType": "$itemType" },
pipeline: [
{ $project: { _id: 1, title: 1 }},
{ $match: { $expr: { $eq: ["$$itemType", "car"] } }}
],
as: 'data'
}
},
{ $lookup:
{
from: 'laptops',
localField: 'itemId',
foreignField: '_id',
let: { "itemType": "$itemType" },
pipeline: [
{ $project: { _id: 1, title: 1 }},
{ $match: { $expr: { $eq: ["$$itemType", "laptop"] } }}
],
as: 'data'
}
}
]);
The problem is, in the result all data fields are empty as data: [].
The syntax seems correct to me. What's wrong?

Answer 1

Any subsequent reassignment of field values will eliminate any previous value.
So, for your aggregation pipeline, you need to assign different values to each "$lookup" "as" field.
For example:
// ...
{ $lookup:
{
from: 'books',
// ...
as: 'booksData'
}
},
{ $lookup:
{
from: 'cars',
// ...
as: 'carsData'
}
},
{ $lookup:
{
from: 'laptops',
// ...
as: 'laptopsData'
}
},
// ...