Mongodb - count from a sub collection inside aggregate lookup - mongodb

Question

I have the following mongo db query that works:
.collection('commentsPosts')
.aggregate(
[
{$match:{
commentedAt: {
$lte: from,
},
...(postId && { postId }),
}
},
{$lookup:
{
from:"users",
localField:"commentedBy",
foreignField:"_id",
as:"commenterDetails"
// NEED TO LOOK UP "posts" collection here to find number of posts by commenterDetails.userId = posterId in posts table
},
}
]
)
I need to go one more level down to TO LOOK UP count of posts to find number of posts where commenterDetails.userId = posterId in posts collection
I am looking for a way to add the working query below to the query above.
const userPostCount = await req.db.collection('posts').countDocuments({
creatorId: userId,
});

Answer 1

You can use nested $lookups like this: https://mongoplayground.net/p/_il8aiilLh5
db.comments.aggregate([
{$lookup:
{
from:"users",
let: {userId: "$commentedBy"},
as:"commenterDetails",
pipeline: [
{$match: {$expr: {
$eq: ["$$userId", "$_id"],
}}},
{$lookup: {
from: "posts",
as: "posts",
let: {posterId: "$_id"},
pipeline: [
{$match: {$expr: {
$eq: ["$posterId", "$$posterId"]
}}},
{$count: "total"},
]
}},
{$addFields: {
posts: {$arrayElemAt: ['$posts', 0]}
}},
]
},
},
{$addFields: {
commenterDetails: {$arrayElemAt: ['$commenterDetails', 0]}
}}
])