How to query an array and retrieve it from MongoDB - mongodb

Question

Updated:
I have a document on the database that looks like this:
My question is the following:
How can I retrieve the first 10 elements from the friendsArray from database and sort it descending or ascending based on the lastTimestamp value.
I don't want to download all values to my API and then sort them in Python because that is wasting my resources.
I have tried it using this code (Python):
listOfUsers = db.user_relations.find_one({'userId': '123'}, {'friendsArray' : {'$orderBy': {'lastTimestamp': 1}}}).limit(10)
but it just gives me this error pymongo.errors.OperationFailure: Unknown expression $orderBy
Any answer at this point would be really helpful! Thank You!

Answer 1

use aggregate
first unwind
then sort according timestap
group by _id to create sorted array
use addfields and filter for getting first 10 item of array
db.collection.aggregate([
{ $match:{userId:"123"}},
{
"$unwind": "$friendsArray"
},
{
$sort: {
"friendsArray.lastTimeStamp": 1
}
},
{
$group: {
_id: "$_id",
friendsArray: {
$push: "$friendsArray"
}
},
},
{
$addFields: {
friendsArray: {
$filter: {
input: "$friendsArray",
as: "z",
cond: {
$lt: [
{
$indexOfArray: [
"$friendsArray",
"$$z"
]
},
10
]
}// 10 is n first item
}
}
},
}
])
https://mongoplayground.net/p/2Usk5sRY2L2
and for pagination use this
db.collection.aggregate([
{ $match:{userId:"123"}},
{
"$unwind": "$friendsArray"
},
{
$sort: {
"friendsArray.lastTimeStamp": 1
}
},
{
$group: {
_id: "$_id",
friendsArray: {
$push: "$friendsArray"
}
},
},
{
$addFields: {
friendsArray: {
$filter: {
input: "$friendsArray",
as: "z",
cond: {
$and: [
{
$gt: [
{
$indexOfArray: [
"$friendsArray",
"$$z"
]
},
10
]
},
{
$lt: [
{
$indexOfArray: [
"$friendsArray",
"$$z"
]
},
20
]
},
]
}// 10 is n first item
}
}
},
}
])

Answer 2

The translation of your find to aggregation(we need unwind that why aggregation is used) would be like the bellow query.
Test code here
Query (for descending replace 1 with -1)
db.collection.aggregate([
{
"$match": {
"userId": "123"
}
},
{
"$unwind": {
"path": "$friendsArray"
}
},
{
"$sort": {
"friendsArray.lastTimeStamp": 1
}
},
{
"$limit": 10
},
{
"$replaceRoot": {
"newRoot": "$friendsArray"
}
}
])
If you want to skip some before limit add one stage also
{
"$skip" : 10
}
To take the 10-20 messages for example.