Get chats list MongoDB - database

Question

In my chat app (+20m users for example) i need to get last updated chats but there's no optimal solution.
Chats
{
id: 1,
title: 'My Group',
updated_at: 137974654
},
{
id: 2,
title: 'Gamers',
updated_at: 137973654
}
Members
{
chat_id: 1,
user_id: 'A'
},
{
chat_id: 2,
user_id: 'B'
}
I don't want to embed members because a user can join thousands of chats so document size is beyond 16MB.
The maximum BSON document size is 16 megabytes.
A potential problem with the embedded document pattern is that it can lead to large documents, especially if the embedded field is unbounded. In this case, you can use the subset pattern to only access data which is required by the application, instead of the entire set of embedded data.
https://docs.mongodb.com/manual/tutorial/model-embedded-one-to-many-relationships-between-documents/#subset-pattern
My solution but not efficient
I first tried to get a list of chat_ids then query chats:
const members = Members.find({ user_id: 'A' }).project({ chat_id: 1 });
const chat_ids = members.map(item => item.chat_id);
const chats = Chats.find({
id: {
$in: chat_ids
}
}).sort({ updated_at: -1 }).limit(20);
But as i said, What if a user has joined +100000 chats? so the first query is too large + the last query is too slow.
What should i do? Do i need a relational database?

Answer 1

Making some assumptions that you are only interested in user_id: 'A', and keeping in mind that I am a MongoDB noob, and I only tested this with the example data you provided, does this do what you want?
db.members.aggregate([
{
$match: {
user_id: "A"
}
},
{
$lookup: {
from: "chats",
localField: "chat_id",
foreignField: "id",
as: "thechats"
}
},
{
"$unwind": "$thechats"
},
{
"$replaceRoot": {
"newRoot": "$thechats"
}
},
{
"$sort": {
updated_at: -1
}
},
{
"$limit": 20
}
])
Try it at mongoplayground.net.