How to fetch the last n months record(consider only the last entry per month) in MongoDB

I am new to MongoDb. I need help to fetch the last n month record, there might be multiple entry per month but the query needs to return only the last entry per month.

For e.g lets say if n is 3 and userId is userId1 (that means return last 3 month record for userId1).

Sample inputs in the collection:

[
 {
   "_id": objectId("aaaaaa"),
   "userId": "userId1",
   "processedAt": "2021-06-01T12:16:49.349Z"
 },
 {
   "_id": objectId("bbbbb"),
   "userId": "userId1",
   "processedAt": "2021-10-11T12:16:49.349Z"
 },
 {
   "_id": objectId("ccccc"),
   "userId": "userId1",
   "processedAt": "2021-10-25T12:16:49.349Z"
 },
 {
   "_id": objectId("eeeee"),
   "userId": "userId1",
   "processedAt": "2021-09-12T12:16:49.349Z"
 },
 {
   "_id": objectId("fffff"),
   "userId": "userId1",
   "processedAt": "2021-09-28T12:16:49.349Z"
 },
 {
   "_id": objectId("ggggg"),
   "userId": "userId1",
   "processedAt": "2021-09-23T12:16:49.349Z"
 },
 {
   "_id": objectId("hhhhh"),
   "userId": "userId1",
   "processedAt": "2021-07-23T12:16:49.349Z"
 },
 {
   "_id": objectId("iiiii"),
   "userId": "userId2",
   "processedAt": "2021-09-29T12:16:49.349Z"
 },
 {
   "_id": objectId("jjjjj"),
   "userId": "userId1",
   "processedAt": "2022-01-29T12:16:49.349Z"
 },
 {
   "_id": objectId("kkkkk"),
   "userId": "userId1",
   "processedAt": "2022-02-29T12:16:49.349Z"
 }, 
]

Expected Result: Should return by userId, limit n months(fetch only the last saved entry of the month) and the ascending order of the month of processedAt:

[{
    "_id": objectId("ccccc"),
    "userId": "userId1",
    "processedAt": "2021-10-25T12:16:49.349Z"
},
{
    "_id": objectId("jjjjj"),
    "userId": "userId1",
    "processedAt": "2022-01-29T12:16:49.349Z"
  },
  {
    "_id": objectId("kkkkk"),
    "userId": "userId1",
    "processedAt": "2022-02-29T12:16:49.349Z"
  }
]

I have tried below query however which is returning all the records. I want query needs to consider only the last entry per month. I have been using mongojs driver v4.1.2

db.collection(collection_name)
                .find({ userId: userId }, { projection: { _id: 0 } })
                .sort({ processedAt: -1 })
                .limit(n)
                .toArray()

Starting from MongoDB 5.0,

You can use $setWindowFields to aggregate a "rank" for the "partition" / "group" (i.e. the month in your example) and only choose the document with top rank.

The ranking can be defined as processedAt: -1 as you want to keep only the latest record in the month with highest rank.

{
    "$setWindowFields": {
      "partitionBy": {
        "$dateToString": {
          "date": "$processedAt",
          "format": "%Y-%m"
        }
      },
      "sortBy": {
        "processedAt": -1
      },
      "output": {
        "rank": {
          $rank: {}
        }
      }
    }
  }

Here is the Mongo playground for your reference.

For MongoDB 3.6+,

As the sample dataset is using ISODate format, it is possible to sort and group the field by leftmost 7 characters (i.e. yyyy-MM). Keeping only the first document inside the month group should do the tricks.

{
    $sort: {
      processedAt: -1
    }
  },
  {
    "$addFields": {
      "month": {
        "$substrCP": [
          "$processedAt",
          0,
          7
        ]
      }
    }
  },
  {
    $group: {
      _id: "$month",
      last: {
        $first: "$$ROOT"
      }
    }
  }

Here is the Mongo playground.