aggregate in mongodb left join with $lookup - mongodb-query

Question

I have three collections
posts=[
{
"id": "p1",
"title": "title 1"
},
{
"id": "p2",
"title": "title 2"
}]
users = [
{
"id": "u1",
"name": "name1"
},
{
"id": "u2",
"name": "name2"
}]
comments = [
{
"userId": "u1",
"postId": "p1",
"comment": "comment 1"
}]
I want to get all collection posts and comments in each post by userId(u1) as:
posts=[
{
"id": "p1",
"title": "title 1",
"comments":[
"userId": "u1",
"comment": "comment 1"
]
},
{
"id": "p2",
"title": "title 2",
"comments":[]
}]
I used aggregate function and $lookup operator but I don't know using the $match operator to filter userId. I used aggregate bellow:
self.db.posts.aggregate([
{
"$lookup":{
"from": "comments",
"localField": "id",
"foreignField": "postId",
"as": "comments",
}
},
{
"$match":{
"comments.userId": {"$eq": param.objectUserId}
},
},
{"$skip": (param.page - 1) * param.pageSize},
{"$limit": param.pageSize},
{"$sort": {"unixDate": pymongo.DESCENDING}}
])
It only return one post in array corresponding with userId="u1"
Please help me!
Thank all!

Answer 1

You have to make use of the pipeline option of $lookup stage and pass the additional conditions that you want to apply.
db.posts.aggregate([
{
"$lookup": {
"from": "comments",
"let": {
"pId": "$id"
},
"pipeline": [
{
"$match": {
"$expr": {
"$eq": [
"$postId",
"$$pId"
],
},
"userId": "u1",
},
},
{
"$project": {
"_id": 0,
"userId": 1,
"comment": 1,
},
},
],
"as": "comments"
}
}
])
Mongo Playground Sample Execution
self.db.posts.aggregate([
{
"$lookup": {
"from": "comments",
"let": {
"pId": "$id"
},
"pipeline": [
{
"$match": {
"$expr": {
"$eq": [
"$postId",
"$$pId"
],
},
"userId": param.objectUserId,
},
},
{
"$project": {
"_id": 0,
"userId": 1,
"comment": 1,
},
},
],
"as": "comments"
}
},
{"$skip": (param.page - 1) * param.pageSize},
{"$limit": param.pageSize},
{"$sort": {"unixDate": pymongo.DESCENDING}}
])