Mongodb aggregation framework and nested $lookup - mongodb

Question

I'm trying to do a nested population by using the mongodb's aggregation framework and I have some troubles with it.
This is the initial data that I have:
[
{
_id: ObjectId('123'),
profileId: ObjectId('456')
},
// ...other documents
]
I apply this aggregation pipeline to it so I can populate the profile field by using the profileId field:
MyModel.aggregate([
{
$lookup: {
from: 'profiles',
localField: 'profileId',
foreignField: '_id',
as: 'profile'
}
},
{
$project: {
profile: {
$arrayElemAt: [ '$profile', 0 ]
},
profileId: 1,
}
},
]
And this is the result:
[
{
_id: ObjectId('123'),
profileId: ObjectId('456'),
profile: {
grades: [
ObjectId('...'),
ObjectId('...')
]
// ... other props
}
}
// ...other documents
]
Now I expand the pipeline by adding a $lookup to populate the grades inside each profile document:
MyModel.aggregate([
{
$lookup: {
from: 'profiles',
localField: 'profileId',
foreignField: '_id',
as: 'profile'
}
},
{
$project: {
profile: {
$arrayElemAt: [ '$profile', 0 ]
},
profileId: 1,
}
},
{
$lookup: {
from: 'profile-grades',
localField: 'profile._id',
foreignField: 'profile',
as: 'profile.grades'
}
}
]
This is the result so far:
[
{
_id: ObjectId('123'),
profileId: ObjectId('456'),
profile: {
grades: [
{
studyLocationId: ObjectId('789'),
// ... other props
},
// ... other grades
]
}
}
// ...other documents
]
The final step that I'm not able to achieve is the population of each studyLocation inside the grades array.
Wanted result:
[
{
_id: ObjectId('123'),
profileId: ObjectId('456'),
profile: {
grades: [
{
studyLocationId: ObjectId('789'),
studyLocation: {
_id: ObjectId('...'),
// other props
},
},
// ... other grades
]
}
}
// ...
]
I've tried by adding another $lookup stage but without luck:
{
$lookup: {
from: 'study-locations',
localField: 'profile.grades.studyLocationId',
foreignField: '_id',
as: 'profile.grades.studyLocation'
}
}
This simply returns:
grades: {
studyLocation: []
}
How should I do it? Is this even possible?
Thank you!

Answer 1

If you are using mongodb3.4 this will work, for previous versions you need unwind operator on grades.