Nested Lookup and filtering - mongodb

Question

I have below collections
User
[
{
id : 'acd-1234',
name : 'some name',
profile_id : 1,
is_graduate: true,
children: [
{ class: 'User', id: 'abcd-123'},
{ class: 'User', id: 'bcd-33'}
]
},
{
id: 'abcd-123',
name : 'jhon',
profile_id : 2
is_graduate: true,
},
{
id: 'bcd-123',
name : 'jhon due',
profile_id : 3,
is_graduate: false
}
]
Profile
[
{
id: 1,
address: 'some address'
},
{
id: 2,
address: 'some other address'
},
{
id: 3,
address: 'some other other address'
}
]
Final Output that i need is ( Parent with only graduate children)
[
{
id: 'acd-1234',
name: 'some name',
is_graduate: true,
profile : {
id: 1,
address: "some address"
},
children: [
{
id: "abcd-123",
name: "jhon",
is_graduate: true,
profile: {
id: 2,
address: "some other address"
}
}
]
}
]
Where i am really stuck is
Making nested lookup. Showing profile with all the children
applying filtering on the childrens
Below error from mongo is not allowing me to use pipeline with localField
$lookup with 'pipeline' may not specify 'localField' or 'foreignField'

Answer 1

Try this:
db.users.aggregate([
{
$lookup: {
from: "profile",
let: { id: "$profile_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$id", "$$id"] }
}
}
],
as: "profile"
}
},
{ $unwind: "$profile" },
{
$lookup: {
from: "users",
let: {
id: { $ifNull: ["$children.id", []] }
},
pipeline: [
{
$match: {
$expr: { $in: ["$id", "$$id"] }
}
},
{
$lookup: {
from: "profile",
localField: "profile_id",
foreignField: "id",
as: "profile"
}
},
{ $unwind: "$profile" }
],
as: "children"
}
}
]);
If you want to show only non empty children then add below $match stage after last $lookup:
{
$match: {
$expr: {
$gt: [{ $size: "$children" }, 0]
}
}
}