mongodb:complex nested aggregation - mongodb

Question

I have this collection:
[
{
"_id": {
"$oid": "60b22e1dbd46fa18a8308318"
},
"title": "basketball",
"price": 12,
"category": "Furniture",
"description": "",
"images": [
"http://res.cloudinary.com/hadarush100/image/upload/v1622289949/nfg948x3zro6gbiuknrz.jpg"
],
"categoryId": 1,
"userId": "60ad16493062eb11141d4927",
"createdAt": 1622289948232,
"chats": [
{
"id": 1,
"createdAt": 1622289948232,
"messages": [
{
"id": "1",
"createdAt": 1622289948232,
"senderId": "60ad16493062eb11141d4927",
"text": "Hello, Im the seller of this product."
}
]
},
{
"id": "2",
"createdAt": 1622289948232,
"messages": [
{
"id": 1,
"createdAt": 1622289948232,
"senderId": "60ad16493062eb11141d4927",
"text": "Hello, Im the seller of this product."
}
]
}
]
}
]
and i want to find specific document (by _id), then dive into specific chat in this document (by id), than use $lookup for replacing the "senderId" property in each message with a "sender" property that contains the full sender details (as a user), that exist in another collection (users). the result needs to look like this:
[
{
"_id": {
"$oid": "60b22e1dbd46fa18a8308318"
},
"title": "basketball",
"price": 12,
"category": "Furniture",
"description": "",
"images": [
"http://res.cloudinary.com/hadarush100/image/upload/v1622289949/nfg948x3zro6gbiuknrz.jpg"
],
"categoryId": 1,
"userId": "60ad16493062eb11141d4927",
"createdAt": 1622289948232,
"chats": [
{
"id": 1,
"createdAt": 1622289948232,
"messages": [
{
"id": "1",
"createdAt": 1622289948232,
"sender": {
"_id": {
"$oid": "60ad16493062eb11141d4927"
},
"username": "hadar",
"email": "hadarushha#gmail.com",
"profileImgUrl": "https://randomuser.me/api/portraits/men/79.jpg",
"createdAt": 1621956168518
},
"text": "Hello, Im the seller of this product."
}
]
},
{
"id": "2",
"createdAt": 1622289948232,
"messages": [
{
"id": 1,
"createdAt": 1622289948232,
"sender": {
"_id": {
"$oid": "60ad16493062eb11141d4927"
},
"username": "hadar",
"email": "hadarushha#gmail.com",
"profileImgUrl": "https://randomuser.me/api/portraits/men/79.jpg",
"createdAt": 1621956168518
},
"text": "Hello, Im the seller of this product."
}
]
}
]
}
]

Answer 1

You can use this aggregation:
$match to filter only selected document (_id)
$unwind multiple time to transform arrays into objects
$lookup to query external collection (users)
$group in reverse order
I assumed that your collections are more or less like this (next time, post both collections and also an example on a working playground)
db={
"products": [
{
"_id": {
"$oid": "60b22e1dbd46fa18a8308318"
},
"title": "basketball",
"price": 12,
"category": "Furniture",
"description": "",
"images": [
"http://res.cloudinary.com/hadarush100/image/upload/v1622289949/nfg948x3zro6gbiuknrz.jpg"
],
"categoryId": 1,
"userId": "60ad16493062eb11141d4927",
"createdAt": 1622289948232,
"chats": [
{
"id": 1,
"createdAt": 1622289948232,
"messages": [
{
"id": "1",
"createdAt": 1622289948232,
"senderId": "60ad16493062eb11141d4927",
"text": "Hello, Im the seller of this product."
}
]
},
{
"id": "2",
"createdAt": 1622289948232,
"messages": [
{
"id": 1,
"createdAt": 1622289948232,
"senderId": "60ad16493062eb11141d4927",
"text": "Hello, Im the seller of this product."
}
]
}
]
},
{
"_id": {
"$oid": "60b22e1dbd46fa18a8308319"
},
"title": "volleyball",
"price": 8,
"category": "Furniture",
"description": "",
"images": [
"http://res.cloudinary.com/hadarush100/image/upload/v1622289949/nfg948x3zro6gbiuknrz.jpg"
],
"categoryId": 1,
"userId": "60ad16493062eb11141d4927",
"createdAt": 1622289948232,
"chats": [
{
"id": 1,
"createdAt": 1622289948232,
"messages": [
{
"id": "1",
"createdAt": 1622289948232,
"senderId": "60ad16493062eb11141d4927",
"text": "Hello, Im the seller of this product."
}
]
},
{
"id": "2",
"createdAt": 1622289948232,
"messages": [
{
"id": 1,
"createdAt": 1622289948232,
"senderId": "60ad16493062eb11141d4928",
"text": "Hello, Im the seller of this product."
}
]
}
]
}
],
"users": [
{
"_id": {
"$oid": "60ad16493062eb11141d4927"
},
"username": "hadar",
"email": "hadarushha#gmail.com",
"profileImgUrl": "https://randomuser.me/api/portraits/men/79.jpg",
"createdAt": 1621956168518
},
{
"_id": {
"$oid": "60ad16493062eb11141d4928"
},
"username": "test",
"email": "test#gmail.com",
"profileImgUrl": "https://randomuser.me/api/portraits/men/49.jpg",
"createdAt": 1621956168528
},
]
}
And here is the working aggregation:
db.products.aggregate([
{
"$match": {
"_id": {
"$oid": "60b22e1dbd46fa18a8308319"
}
}
},
{
"$unwind": "$chats"
},
{
"$unwind": "$chats.messages"
},
{
"$addFields": {
"chats.messages.senderIdObjId": {
"$convert": {
"input": "$chats.messages.senderId",
"to": "objectId",
}
}
}
},
{
"$lookup": {
"from": "users",
"localField": "chats.messages.senderIdObjId",
"foreignField": "_id",
"as": "chats.messages.sender"
}
},
{
"$unwind": "$chats.messages.sender"
},
{
"$group": {
"_id": "$chats.id",
"messages": {
"$push": "$chats.messages"
},
"allFields": {
"$first": "$$ROOT"
}
}
},
{
"$addFields": {
"allFields.chats.messages": "$messages"
}
},
{
"$replaceWith": "$allFields"
},
{
"$group": {
"_id": "$_id",
"chats": {
"$push": "$chats"
},
"allFields": {
"$first": "$$ROOT"
}
}
},
{
"$addFields": {
"allFields.chats": "$chats"
}
},
{
"$replaceWith": "$allFields"
},
])
Working Playground here