Mongodb to fetch top 100 results for each category - mongodb

Question

I have a collection of transactions that has below schema:
{
_id,
client_id,
billings,
date,
total
}
What I want to achieve is to get the 10 latest transaction models based on the date for a list of client IDs. I don't think the $slice well as the use case is mostly for embedded arrays.
Currently, I am iterating through the client_ids and using find with the limit but it is extremely slow.
UPDATE
Example
https://mongoplayground.net/p/urKH7HOxwqC
This shows two clients with 10 transaction each on different days, I want to write a query that would return latest 5 transaction for each.
Any suggestions of how to query data to make it faster?

Answer 1

The most efficient way would be to just execute multiple queries, 1 for each client, like so:
const clients = await db.collection.distinct('client_id');
const results = await Promise.all(
clients.map((clientId) => db.collection.find({client_id: clientId}).sort({date: -1}).limit(5))
)
To improve this performance make sure you have an index on client_id and date. If for whatever reason you can't built these indexes I'd recommend using this following pipeline (with syntax available starting version 5.3+):
db.collection.aggregate([
{
$group: {
_id: "$client_id",
latestTransactions: {
"$bottomN": {
"n": 5,
"sortBy": {
"date": 1
},
"output": "$$ROOT"
}
}
}
}
])
Mongo Playground