MongoDB: How to make two unions to the same collection with matrices - mongodb

Question

I have user records with posts and posts shared with them, that is, users can share the posts with other users. I need to be able to bring or get only the posts shared with him, using as a reference the id of the user who shared the post and the id of the post.
when I use the user id as a reference, it works, but when I try to combine it with the id of the post it does not get anything, this happens when I try to use only the id of the post to get the shared posts.
This would be the structure of the records
{
"_id" : ObjectId("5cd573b2bb9ad84f9bba2f74"),
"name" : "name 4",
"posts" : [
{
"_id" : ObjectId("5cd573b2bb9ad84f9bba2f72"),
"name" : "post 1"
},
{
"_id" : ObjectId("5cd573b2bb9ad84f9bba2f73"),
"name" : "post 2"
}
],
"postSharedWithMe" : [
{
"user_id" : "5cd4aaedfcf8d8583cf97494",
"post_id" : "5cd4aaedfcf8d8583cf97492"
},
{
"user_id" : "5cd4aaedfcf8d8583cf97494",
"post_id" : "5cd4aaedfcf8d8583cf97493"
}
]
}
and in this way he tried to consult them
db.users.aggregate([
{ "$match": { "_id": ObjectId("5cd573b2bb9ad84f9bba2f74") }},
{ $unwind:"$postSharedWithMe" },
{ $unwind:"$posts" },
{
$lookup:
{
from: "users",
let: {
user_id: { "$toObjectId": "$postSharedWithMe.user_id"},
post_id : { "$toObjectId": "$postSharedWithMe.post_id"}
},
pipeline: [
{ $match:
{ $expr:
{ $and:
[
{ $eq: [ "$_id", "$$user_id" ] },
{ $eq: [ "$posts._id", "$$post_id" ] }
]
}
}
},
],
as: "sharedPosts"
}
},
{ $unwind:"$sharedPosts" },
{ "$group": {
"_id": "$_id",
"sharedPosts": { "$push": "$sharedPosts" }
}
}
])
and this is the result:
Fetched 0 record(s) in 0ms
and this is what I expected
{
"_id" : ObjectId("5cd573b2bb9ad84f9bba2f74"),
"name" : "username",
"posts" : [
{
"_id" : ObjectId("5cd573b2bb9ad84f9bba2f72"),
"name" : "post 1"
},
{
"_id" : ObjectId("5cd573b2bb9ad84f9bba2f73"),
"name" : "post 2"
}
],
"sharedPosts" : [
{
"_id" : ObjectId("id"),
"name" : "shared post"
},
{
"_id" : ObjectId("id"),
"name" : "shared post"
}
]
}

Answer 1

apparently I needed to go through all the posts first while referring to the shared posts, the result of this was an array, now I just needed to make the $ unwind and compare with $ eq and it worked!
db.users.aggregate([
{ $match: { "_id": ObjectId("5cd573b2bb9ad84f9bba2f74") }},
{ $unwind: "$postSharedWithMe" },
{
$lookup:
{
from: "users",
let: {
user_id: { $toObjectId: "$postSharedWithMe.user_id"},
post_id : { $toObjectId: "$postSharedWithMe.post_id"}
},
pipeline: [
{ $match:
{ $expr:
{ $and:
[
{ $eq: [ "$$user_id", "$_id" ] },
{ $in: ["$$post_id", "$posts._id" ] },
]
}
}
},
{ $unwind: "$posts" },
{ $match: { $expr: { $eq: [ "$posts._id", "$$post_id" ] } } },
],
as: "sharedPosts"
}
},
{ $unwind: "$sharedPosts" },
{ $group: {
_id: "$_id",
name: { "$first": "$name" },
posts: { "$first": "$posts" },
sharedPosts: { $push:
"$sharedPosts.posts"
}
}
}
])