I need limited nested array in mongodb document - mongodb

Question

I have a document like
{
"deviceId" : "1106",
"orgId" : "5ffe9fe1c9e77c0006f0aad3",
"values" : [
{
"paramVal" : 105.0,
"dateTime" : ISODate("2021-05-05T09:18:08.000Z")
},
{
"paramVal" : 110.0,
"dateTime" : ISODate("2021-05-05T09:18:08.000Z")
},
{
"paramVal" : 115.0,
"dateTime" : ISODate("2021-05-05T10:18:08.000Z")
},
{
"paramVal" : 125.0,
"dateTime" : ISODate("2021-05-05T11:18:08.000Z")
},
{
"paramVal" : 135.0,
"dateTime" : ISODate("2021-05-05T12:18:08.000Z")
}
]
}
Now I need to filter a document which I can do easily with match or find but in that document the subarray i.e. values should have latest 2 values because in future the count can be more than 100.
the output should be like
{
"deviceId" : "1106",
"orgId" : "5ffe9fe1c9e77c0006f0aad3",
"values" : [
{
"paramVal" : 125.0,
"dateTime" : ISODate("2021-05-05T11:18:08.000Z")
},
{
"paramVal" : 135.0,
"dateTime" : ISODate("2021-05-05T12:18:08.000Z")
}
]
}

Answer 1

Try $slice operator, to select number of elements, pass negative value to select documents from below/last elements,
db.collection.aggregate([
{ $set: { values: { $slice: ["$values", -2] } } }
])
Playground
I need for the array values in sorted order by date
There is no straight way to do this, check the below aggregation query, but it will cause the performance issues, i would suggest to change you schema structure to manage this data order by date,
$unwind deconstruct values array
$sort by dateTime in descending order
$group by _id and reconstruct values array and return other required fields
$slice to select number of elements, pass negative value to select documents from below/last elements
db.collection.aggregate([
{ $unwind: "$values" },
{ $sort: { "values.dateTime": -1 } },
{
$group: {
_id: "$_id",
deviceId: { $first: "$deviceId" },
orgId: { $first: "$orgId" },
values: { $push: "$values" }
}
},
{ $set: { values: { $slice: ["$values", 2] } } }
])
Playground