How to reverse $unwind or re-assemble after $lookup? - arrays

Question

I´ve been trying to reverse $unwind in nested array. Please, if you could help me it would be great. Thanks in advance.
Here are the details:
checklists collection, this collection has steps and each step has many areas, and I'd like to lookup to fill the area by id. I did it but I cannot reverse $unwind.
{
"steps": [{
"name": "paso1",
"description": "paso1",
"estimated_time": 50,
"active": true,
"areas": [{
"area_id": "60b6e728c44f0365c0d547d6"
}, {
"area_id": "60b6e7a2c44f0365c0d547d8"
}]
}, {
"name": "paso2",
"description": "o",
"estimated_time": 7,
"active": true,
"areas": [{
"area_id": "60b6e76ac44f0365c0d547d7"
}]
}, {
"name": "paso2",
"description": "l",
"estimated_time": 7,
"active": true,
"areas": [{
"area_id": "60b6e728c44f0365c0d547d6"
}]
}],
"name": "prueba",
"description": "prueba",
"type": "prueba",
"active": true,
"updated_at": {
"$date": "2021-06-02T23:56:02.232Z"
},
"created_at": {
"$date": "2021-06-01T22:44:57.114Z"
},
"__v": 0
}
area collection
{
"_id":"60b6e706c44f0365c0d547d5"
"name": "Development",
"short_name": "DEV",
"description": "Development area",
"updated_at": {
"$date": "2021-06-02T02:03:50.383Z"
},
"created_at": {
"$date": "2021-06-02T02:03:50.383Z"
},
"__v": 0,
"active": true
}
My aggregation
db.checklists.aggregate([
{
"$unwind": "$steps"
},
{
"$unwind": "$steps.areas"
},
{
"$lookup": {
"from": "areas",
"let": {
"area_id": {
"$toObjectId": "$steps.areas.area_id"
}
},
"pipeline": [
{
"$match": {
"$expr": {
"$eq": [
"$_id",
"$$area_id"
]
}
}
}
],
"as": "convertedItems"
}
},
{
"$group": {
"_id": "$steps.name",
"root": {
"$first": "$$ROOT"
},
"items": {
"$push": {
"$mergeObjects": [
"$steps.areas",
{
"$arrayElemAt": [
"$convertedItems",
0
]
}
]
}
},
}
},
{
"$addFields": {
"values": {
"$reduce": {
"input": "$items",
"initialValue": [],
"in": {
"$concatArrays": [
"$$value",
{
"$cond": [
{
"$in": [
"$$this.area_id",
"$$value.area_id"
]
},
[],
[
"$$this"
]
]
}
]
}
}
}
}
},
{
"$addFields": {
"root.steps.areas": "$values"
}
},
{
"$replaceRoot": {
"newRoot": "$root"
}
},
{
"$group": {
"_id": "$_id",
"root": {
"$first": "$$ROOT"
},
"steps": {
"$push": "$steps"
}
}
},
{
"$addFields": {
"root.steps": "$steps"
}
},
{
"$replaceRoot": {
"newRoot": "$root"
}
},
{
"$project": {
"convertedItems": 0
}
}
])
I don´t get to form this output:
{
"steps": [{
"name": "paso1",
"description": "paso1",
"estimated_time": 50,
"active": true,
"areas": [{
"_id": "60b6e728c44f0365c0d547d6",
"name":"Development",
..... //join or lookup
}, {
"_id": "60b6e7a2c44f0365c0d547d8",
"name":"Development",
..... //join or lookup
}]
}],
"name": "prueba",
"description": "prueba",
"type": "prueba",
"active": true,
"updated_at": {
"$date": "2021-06-02T23:56:02.232Z"
},
"created_at": {
"$date": "2021-06-01T22:44:57.114Z"
},
"__v": 0
}
Thank you very much!

Answer 1

$unwind deconstruct steps array
$lookup with areas collection pass area_id in let
$match to check is _id in area_ids after converting to string
$project to show required fields
$group by _id and reconstruct the steps array and pass your required fields
db.checklists.aggregate([
{ $unwind: "$steps" },
{
$lookup: {
from: "areas",
let: { area_id: "$steps.areas.area_id" },
pipeline: [
{
$match: {
$expr: { $in: [{ $toString: "$_id" }, "$$area_id"] }
}
},
{ $project: { name: 1 } }
],
as: "steps.areas"
}
},
{
$group: {
_id: "$_id",
steps: { $push: "$steps" },
name: { $first: "$name" },
description: { $first: "$description" },
type: { $first: "$type" },
active: { $first: "$active" },
updated_at: { $first: "$updated_at" },
created_at: { $first: "$created_at" },
__v: { $first: "$__v" }
}
}
])
Playground