How to use aggregate $lookup in nested data from mongoDB? - database

Question

I have collections like this:
// tasks
[
{_id: '123', _user: '345', _solutions: ['567', '678'] }
]
// solutions
[
{ _id: '567', _task: '123', _user: '345' },
{ _id: '678', _task: '123', _user: '345' }
]
// users
[
{ _id: '345', name: 'Tom' }
]
With this code:
await db
.collection<Task>('tasks')
.aggregate([
{ $match: { _id: id } },
{
$lookup: {
from: 'solutions',
// I guess here should be pipeline
localField: '_solutions',
foreignField: '_id',
as: '_solutions',
},
},
{ $lookup: { from: 'users', localField: '_user', foreignField: '_id', as: '_user' } },
])
I have result as below:
task = {
_id: 5e14e877fa42402079e38e44,
_solutions: [
{
_id: 5e15022ccafcb4869c153e61,
_task: 5e14e877fa42402079e38e44,
_user: 5e007403fd4ca4f47df69913, <-- this should be userObject instead
},
{
_id: 5e164f31cafcb4869c153e62,
_task: 5e14e877fa42402079e38e44,
_user: 5e007403fd4ca4f47df69913, <-- this should be userObject instead
}
],
_user: [
{
_id: 5e007403fd4ca4f47df69913,
_solutions: [Array],
_tasks: [Array],
}
]
}
and I don't know how to $lookup into _solutions._user - so instead of objectId I will have exact user object.

Answer 1

You can run $lookup with custom pipeline for outer lookup and the regular one for users:
db.tasks.aggregate([
{
$match: {
_id: "123"
}
},
{
$lookup: {
from: "solutions",
let: { solutions: "$_solutions" },
pipeline: [
{ $match: { $expr: { $in: [ "$_id", "$$solutions" ] } } },
{
$lookup: {
from: "users",
localField: "_user",
foreignField: "_id",
as: "_user"
}
},
{ $unwind: "$_user" }
],
as: "_solutions",
}
}
])
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