Tips for choosing the most recent entry for each category in a MongoDB database using typescript and mongoose

My mongoDB collection is structured like this:

[
 {user: "joe",  event: "foo", timestamp: 1},
 {user: "meg",  event: "blah", timestamp: 2},
 {user: "rick", event: "bork", timestamp: 3},
 {user: "joe",  event: "bing", timestamp: 4},
 {user: "meg",  event: "biz",  timestamp: 5},
 {user: "tim",  event: "boz",  timestamp: 6}
]

I am aiming to identify the most recent event for each user, resulting in a set of data as follows:

[
 {user: "rick", event: "bork", timestamp: 3},
 {user: "joe",  event: "bing", timestamp: 4},
 {user: "meg",  event: "biz",  timestamp: 5},
 {user: "tim",  event: "boz",  timestamp: 6}
]

Although I have reviewed the mongoose aggregation framework documentation, I haven't found a direct solution. Many examples group by user but lose some details in the process. I am unsure how to preserve the complete documents at the end.

javascripttypescriptmongodbmongoose

Answer №1

  1. $setWindowFields - Group the data by the user field, order each group by timestamp in descending order, and assign a rank to each document using the $rank operator.

  2. $match - Filter the documents where rank: 1, selecting only the latest timestamp for each user.

  3. $sort - Sort the documents by timestamp in ascending order.

  4. $unset - Remove the rank field from the output.

db.collection.aggregate([
  {
    $setWindowFields: {
      partitionBy: "$user",
      sortBy: {
        timestamp: -1
      },
      output: {
        rank: {
          $rank: {}
        }
      }
    }
  },
  {
    $match: {
      rank: 1
    }
  },
  {
    $sort: {
      timestamp: 1
    }
  },
  {
    , $unset: "rank"
  }
])

Check out the demo on Mongo Playground

Answer №2

If you combine the sort, group, and project functions, I have a solution available

db.collection.aggregate([
  { $sort: { timestamp: -1 } },

  {
    $group: {
      _id: '$user',
      doc: { $first: '$$ROOT' },
    },
  },
  {
    $project: {
      _id: 0,
      user: '$doc.user',
      event: '$doc.event',
      timestamp: '$doc.timestamp',
    },
  },
])

This method allows you to retain all document information during the process

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