Unable to sort documents after grouping mongodb aggregation framework

[Ali_Abyer]

so i am trying to sort notifications array of user by it’s insertion date (i want the latest one on top) but it seems to be not working am i missing something ?

here’s the template for data:

{
  _id: "628ceeae3df06d49419f0bb4",
  name: "John",
  notifications: [
    {_id: "someIdA", details: "xyz", dateTime: "1653321337762"},
    {_id: "someIdB", details: "jkl", dateTime: "1653321337762"}
    {_id: "someIdC", details: "abc", dateTime: "1653321321323"}
    {_id: "someIdD", details: "lmn", dateTime: "1653123412341"}
  ]
}

and the aggregation pipeline that i’m trying:

const foundUser = await users.aggregate([
{
$match: {
_id: mongoose.Types.ObjectId(userId)
}
},
{
$project: {
notifications: 1,
_id: 0
}
},
{
$sort: {
_id: -1
}
}
])

[steevej (Steeve Juneau)]

If you remove your $sort you will see that documents after the $project look like:

notifications: [
    {_id: "someIdA", details: "xyz", dateTime: "1653321337762"},
    {_id: "someIdB", details: "jkl", dateTime: "1653321337762"}
    {_id: "someIdC", details: "abc", dateTime: "1653321321323"}
    {_id: "someIdD", details: "lmn", dateTime: "1653123412341"}
  ]

That is, all fields projected out except for notifications. So as you can see, your $sort:{_id:-1} does not make sense because you do not have a field named _id anymore.

If you really want to sort on the top level _id:628ceeae3df06d49419f0bb4, you need to remove _id:0 from your $project.

But, if you want to sort based on the _id within the notifications array and keep the result as an array within each top level document, you will need to use $sortArray.

If you do not want a sorted array within a top level document, you will need an $unwind stage before the $sort stage.

The exact scenario depends on your desired result. If you publish expected resulting documents, the help will be more precise.

[Ali_Abyer]

alright it’s done, so the problem was exactly the not use of $unwind