MongoDB Aggregation - How to get upper boundary in $bucket stage?

Let's say one has data below:

[
  { "_id" : 1, "last_name" : "Bernard", "first_name" : "Emil", "year_born" : 1868, "year_died" : 1941, "nationality" : "France" },
  { "_id" : 2, "last_name" : "Rippl-Ronai", "first_name" : "Joszef", "year_born" : 1861, "year_died" : 1927, "nationality" : "Hungary" }
]

Then runs $bucket stage :

db.artists.aggregate( [
  {
    $bucket: {
      groupBy: "$year_born",                        // Field to group by
      boundaries: [ 1840, 1850, 1860, 1870, 1880 ], // Boundaries for the buckets
      default: "Other",                             // Bucket ID for documents which do not fall into a bucket
      output: {                                     // Output for each bucket
        "count": { $sum: 1 },
        "artists" :
          {
            $push: {
              "name": { $concat: [ "$first_name", " ", "$last_name"] },
              "year_born": "$year_born"
            }
          }
      }
    }
  }
] );

The output is like this:

{ 
  "_id" : 1860, 
  "count" : 2, 
  "artists" : [ 
    { "name" : "Emil Bernard", "year_born" : 1868 },
    { "name" : "Joszef Rippl-Ronai", "year_born" : 1861 } 
  ] 
}

Example reference: MongoDB $bucket aggregation

-- Question --

Lower boundary is 1860 which is represented in the _id field, but how to get the upper boundary which is 1870?

Thank you!

There's no Mongo-y way to do this.

If the years are hard coded you can just add this custom logic with an $addFields, like so:

  {
    $addFields: {
      upperBound: {
        $sum: [
          "$_id",
          10
        ]
      }
    }
  }

Mongo Playground

If they are passed dynamically you can use a similar approach and just extract the next element in the array:

const boundaries: [ 1840, 1850, 1860, 1870, 1880 ]

db.collection.aggregate([
  {
    $bucket: {
      groupBy: "$year_born",
      // Field to group by
      boundaries: boundaries,
      // Boundaries for the buckets
      default: "Other",
      // Bucket ID for documents which do not fall into a bucket
      output: {
        // Output for each bucket
        "count": {
          $sum: 1
        },
        "artists": {
          $push: {
            "name": {
              $concat: [
                "$first_name",
                " ",
                "$last_name"
              ]
            },
            "year_born": "$year_born"
          }
        }
      }
    }
  },
  {
    $addFields: {
      upperBound: {
        "$arrayElemAt": [
          boundaries,
          {
            $sum: [
              {
                "$indexOfArray": [
                  boundaries,
                  "$_id"
                ]
              },
              1
            ]
          }
        ]
      }
    }
  }
])

Mongo Playground