Stucking with nested array lookup in MongoDB - mongodb

Question

I'm trying to lookup query from nested array in mongodb and I'm getting stuck.
I have total threee collections.
(1) Channel (Parent)
(2) ChannelThreads (Children)
(3) Users
Channel Collection:
{
"_id" : ObjectId("61efcbdc1aa27f83da47c93f"),
"tags" : [],
"slug_history" : [
"iny1Xik"
],
"title" : "Pirate Chat",
"settingId" : ObjectId("61408586b719c8ce89f08674"),
"status" : "published",
"lockedPageContent" : "",
"slug" : "iny1Xik",
"createdAt" : ISODate("2022-01-25T10:07:24.144Z"),
"updatedAt" : ISODate("2022-01-25T10:07:24.144Z"),
"__v" : 0
}
Channel Thread Collection:
{
"_id" : ObjectId("61efcd5df82318884746eb80"),
"threadImage" : [],
"parentId" : null,
"channelId" : ObjectId("61efcbdc1aa27f83da47c93f"),
"authorId" : ObjectId("6177de8f8a5fd72a4f37b7db"),
"threadText" : "New Message",
"reactions" : [
{
"authors" : [
ObjectId("3687de8f8a5fd72a4f37b7bg")
],
"_id" : ObjectId("61ef856432753c196382c37d"),
"icon" : "&#128528"
}
],
"createdAt" : ISODate("2022-01-25T10:13:49.033Z"),
"updatedAt" : ISODate("2022-01-25T10:13:49.033Z"),
"__v" : 0
}
User Collection:
{
"_id" : ObjectId("6177de8f8a5fd72a4f37b7db"),
"image" : "",
"tags" : [],
"pushTokens" : [],
"lastLogin" : ISODate("2022-01-25T10:08:19.055Z"),
"firstName" : "dinesh",
"lastName" : "patel",
"email" : "dineshpatel#example.com",
"infusionSoftId" : "784589",
"role" : "user",
"__v" : 0,
"settings" : {
"commentNotification" : false,
"commentReplyNotification" : true
}
}
I'm trying to implement lookup for authors of thread reactions.
Expected Output:
{
"_id": ObjectId("61efcbdc1aa27f83da47c93f"),
"tags": [],
"slug_history": [
"iny1Xik"
],
"title": "Pirate Chat",
"settingId": ObjectId("61408586b719c8ce89f08674"),
"status": "published",
"lockedPageContent": "",
"slug": "iny1Xik",
"createdAt": ISODate("2022-01-25T10:07:24.144Z"),
"updatedAt": ISODate("2022-01-25T10:07:24.144Z"),
"__v": 0,
"threads": [
{
"_id": ObjectId("61efcd5df82318884746eb80"),
"threadImage": [],
"parentId": null,
"channelId": ObjectId("61efcbdc1aa27f83da47c93f"),
"authorId": {
"_id": ObjectId("6177de8f8a5fd72a4f37b7db"),
"image": "",
"tags": [],
"pushTokens": [],
"lastLogin": ISODate("2022-01-25T10:08:19.055Z"),
"firstName": "dinesh",
"lastName": "patel",
"email": "dineshpatel#example.com",
"infusionSoftId": "something",
"role": "user",
"__v": 0,
"settings": {
"commentNotification": false,
"commentReplyNotification": true
}
},
"threadText": "New Message",
"reactions": [
{
"authors": [
{
"_id": ObjectId("3687de8f8a5fd72a4f37b7bg"),
"image": "",
"tags": [],
"pushTokens": [],
"lastLogin": ISODate("2022-01-25T10:08:19.055Z"),
"firstName": "kayle",
"lastName": "hell",
"email": "kylehell#example.com",
"infusionSoftId": "8475151",
"role": "user",
"__v": 0,
"settings": {
"commentNotification": false,
"commentReplyNotification": true
}
}
],
"_id": ObjectId("61ef856432753c196382c37d"),
"icon": "&#128528"
}
],
"createdAt": ISODate("2022-01-25T10:13:49.033Z"),
"updatedAt": ISODate("2022-01-25T10:13:49.033Z"),
"__v": 0
}
]
}
How can write lookup query for reaction authors.
Thanks in advance!!

Answer 1

You can try nested lookup,
$lookup with channel thread collection, pass channel id in let
$match to match channelId condition
$lookup with user collection to get author info for authorId
$lookup with user collection to get reactions's authors info
$arrayElemAt to get first element from authorId
$map to iterate loop of reactions array, $filter to iterate loop of users and get matching author user info from users array,
$mergeObjects to merge authors and current object properties
$$REMOVE to remove users field because it is not needed now
db.channel.aggregate([
{
$lookup: {
from: "channelThread",
let: { channelId: "$_id" },
pipeline: [
{ $match: { $expr: { $eq: ["$$channelId", "$channelId"] } } },
{
$lookup: {
from: "user",
localField: "authorId",
foreignField: "_id",
as: "authorId"
}
},
{
$lookup: {
from: "user",
localField: "reactions.authors",
foreignField: "_id",
as: "users"
}
},
{
$addFields: {
authorId: { $arrayElemAt: ["$authorId", 0] },
reactions: {
$map: {
input: "$reactions",
as: "r",
in: {
$mergeObjects: [
"$$r",
{
authors: {
$filter: {
input: "$users",
cond: { $in: ["$$this._id", "$$r.authors"] }
}
}
}
]
}
}
},
users: "$$REMOVE"
}
}
],
as: "threads"
}
}
])
Playground