Perform $lookup with object value in array from pipeline - node.js

Question

So I have 3 models user, property, and testimonials.
Testimonials have a propertyId, message & userId. I've been able to get all the testimonials for each property with a pipeline.
Property.aggregate([
{ $match: { _id: ObjectId(propertyId) } },
{
$lookup: {
from: 'propertytestimonials',
let: { propPropertyId: '$_id' },
pipeline: [
{
$match: {
$expr: {
$and: [{ $eq: ['$propertyId', '$$propPropertyId'] }],
},
},
},
],
as: 'testimonials',
},
},
])
The returned property looks like this
{
.... other property info,
testimonials: [
{
_id: '6124bbd2f8eacfa2ca662f35',
userId: '6124bbd2f8eacfa2ca662f29',
message: 'Amazing property',
propertyId: '6124bbd2f8eacfa2ca662f2f',
},
{
_id: '6124bbd2f8eacfa2ca662f35',
userId: '6124bbd2f8eacfa2ca662f34',
message: 'Worth the price',
propertyId: '6124bbd2f8eacfa2ca662f2f',
},
]
}
User schema
firstName: {
type: String,
required: true,
},
lastName: {
type: String,
required: true,
},
email: {
type: String,
required: true,
},
Property schema
name: {
type: String,
required: true,
},
price: {
type: Number,
required: true,
},
location: {
type: String,
required: true,
},
Testimonial schema
propertyId: {
type: ObjectId,
required: true,
},
userId: {
type: ObjectId,
required: true,
},
testimonial: {
type: String,
required: true,
},
Now the question is how do I $lookup the userId from each testimonial so as to show the user's info and not just the id in each testimonial?
I want my response structured like this
{
_id: '6124bbd2f8eacfa2ca662f34',
name: 'Maisonette',
price: 100000,
testimonials: [
{
_id: '6124bbd2f8eacfa2ca662f35',
userId: '6124bbd2f8eacfa2ca662f29',
testimonial: 'Amazing property',
propertyId: '6124bbd2f8eacfa2ca662f34',
user: {
_id: '6124bbd2f8eacfa2ca662f29',
firstName: 'John',
lastName: 'Doe',
email: 'jd#mail.com',
}
},
{
_id: '6124bbd2f8eacfa2ca662f35',
userId: '6124bbd2f8eacfa2ca662f27',
testimonial: 'Worth the price',
propertyId: '6124bbd2f8eacfa2ca662f34',
user: {
_id: '6124bbd2f8eacfa2ca662f27',
firstName: 'Sam',
lastName: 'Ben',
email: 'sb#mail.com',
}
}
]
}

Answer 1

You can put $lookup stage inside pipeline,
$lookup with users collection
$addFields, $arrayElemAt to get first element from above user lookup result
Property.aggregate([
{ $match: { _id: ObjectId(propertyId) } },
{
$lookup: {
from: "propertytestimonials",
let: { propPropertyId: "$_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$propertyId", "$$propPropertyId"] }
}
},
{
$lookup: {
from: "users",
localField: "userId",
foreignField: "_id",
as: "user"
}
},
{
$addFields: {
user: { $arrayElemAt: ["$user", 0] }
}
}
],
as: "testimonials"
}
}
])
Playground