What's the best way to using nosql in this situation? - mongodb

Question

I want to find the most up-to-date data by user in mongodb.
Assuming the data exists as follows:
{
_id: "1",
user_id: "userA",
date: "2022-10-20 11:00:00.000000000"
},
{
_id: "2",
user_id: "userA",
date: "2022-10-25 18:00:00.000000000"
},
{
_id: "3",
user_id: "userB",
date: "2022-10-25 18:00:00.000000000"
},
{
_id: "4",
user_id: "userC",
date: "2022-10-25 18:00:00.000000000"
}
So I want find result like this:
{
_id: "2",
user_id: "userA",
date: "2022-10-25 18:00:00.000000000"
},
{
_id: "3",
user_id: "userB",
date: "2022-10-25 18:00:00.000000000"
},
{
_id: "4",
user_id: "userC",
date: "2022-10-25 18:00:00.000000000"
}
What's the best way to this?

Answer 1

If you're using Mongo version 5.2+ then you can use $group with the new $bottom operator, like so:
db.collection.aggregate([
{
$group: {
_id: "$user_id",
root: {
"$bottom": {
"sortBy": {
"date": 1
},
"output": "$$ROOT"
}
}
}
},
{
$replaceRoot: {
newRoot: "$root"
}
}
])
MongoPlayground
Otherwise there is no real good way of doing this, you'll have to sort the entire collection by date (assuming all dates are saved in a proper format) then group by user id and get the "latest" instance.
db.collection.aggregate([
{
$sort: {
date: -1
}
},
{
$group: {
_id: "$user_id",
root: {
$first: "$$ROOT"
}
}
},
{
$replaceRoot: {
newRoot: "$root"
}
}
])
Mongo Playground