MongoDB aggregate fill/replace array of ObjectId - arrays

Question

I have the following MongoDB structure:
campaign:{
_id: '5e4eee638552043e60e5073b',
name: 'Test campaign',
partners: [
{
_id: '5e4fa9fbbdeb4a5878ac2a35',
options: [Object],
partner: '5e4e9bc2101ecb2e8764190c',
},
{
_id: '5e4ff7c6e54b6d676d97b7a6',
options: [Object],
partner: '5e4ff51c23f44266a8add39e',
}
]
}
Using the following aggregate function, I can lookup the ID of each of the partners and add to a new array:
{
$lookup:
{
from: 'campaigns',
let: { campaign: '$campaign' },
pipeline: [
{ $match: { $expr: { $and: [{ $eq: ['$_id', '$$campaign'] }] } } },
{
$lookup:
{
from: 'partners',
let: { partner: '$partner.partner' },
pipeline: [
{ $match: { $expr: { $in: ['$_id', '$$partner'] } } },
{ $project: { _id: 1, name: 1 } }
],
as: 'partner1'
}
},
{
$project: {
_id: 1, shortName: 1, userInterface: 1, options: 1, check: 1, description: 1, partner: 1, partner1: 1 } }
],
as: 'campaign'
}
}
This is new output:
campaign: {
_id: '5e4eee638552043e60e5073b',
name: 'Test campaign',
partners: [
...
],
partner1: [
{ _id: '5e4e9bc2101ecb2e8764190c', name: '123' },
{ _id: '5e4ff51c23f44266a8add39e', name: '456' }
]
}
But what I want to do is have the _id of each element in partner array be replaced with the full partner object.
Partner object:
partner
{
_id: '5e4e9bc2101ecb2e8764190c',
name: '123',
etc: ...
},
{
_id: '5e4ff51c23f44266a8add39e',
name: '456'
etc: ...
}
This is what the final object should look like:
[
{
"campaign": [
{
"partner": [
{
"_id": "5e4eee638552043e60e5073b",
"options": [
{}
],
"partner": {
"_id": "5e4e9bc2101ecb2e8764190c",
"name": "123"
}
}
]
}
]
}
]

Answer 1

You need to add extra 3 stages:
db.collection.aggregate([
//your $lookup,
{
$unwind: "$campaign"
},
{
$project: {
_id: 1,
"campaign.partner": {
$map: {
input: "$campaign.partner",
as: "partner",
in: {
$let: {
vars: {
partner1: {
$filter: {
input: "$campaign.partner1",
cond: {
$eq: [
"$$partner.partner",
"$$this._id"
]
}
}
}
},
in: {
// we merge partner[i] with partner1[j]
$mergeObjects: [
"$$partner",
{
partner: {
$arrayElemAt: [
"$$partner1",
0
]
}
}
]
}
}
}
}
}
}
},
{
$group: {
_id: "$_id",
campaign: {
$push: "$campaign"
}
}
}
])
MongoPlayground