How do get the last substring using the MongoDB aggregation framework?

Question

Let's say I have these documents

> db.coll.find().toArray()
[
    {
        "_id" : ObjectId("5a36c3e218948d0722457078"),
        "locality" : "Nasimi, Baku, Azerbaijan"
    },
    {
        "_id" : ObjectId("5a36c3e218948d0722457079"),
        "locality" : "Garland, TX, USA"
    },
    {
        "_id" : ObjectId("5a36c3e218948d072245707a"),
        "locality" : "Halytskyi District, Lviv, Lviv Oblast, Ukraine"
    },
    {
        "_id" : ObjectId("5a36c3e218948d072245707b"),
        "locality" : "Tozeur, Tunisia"
    }
]

I would like to get only the country, ie whatever is after the last comma ( ', ' ).

eg

[
    "Azerbaijan",
    "USA",
    "Ukraine",
    "Tunisia"
]

I've managed to get the index of the first comma and what is after that but I can't figure out how to to get the last one.

db.coll
  .aggregate([
    {
      $project: {
        lastIndexOf: { $indexOfBytes: [ '$locality', ', ' ] },
        locality: '$locality',
      }
    },
    {
      $project: {
        lastIndexOfPlusTwo: { $add: [ '$lastIndexOf', 2 ] },
        locality: '$locality',
      }
    },
    {
      $project: { country: { $substr: [ '$locality', '$lastIndexOfPlusTwo', -1 ] } }
    }
  ]).pretty()

{
    "_id" : ObjectId("5a36c3e218948d0722457078"),
    "country" : "Baku, Azerbaijan"
}
{
    "_id" : ObjectId("5a36c3e218948d0722457079"),
    "country" : "TX, USA"
}
{
    "_id" : ObjectId("5a36c3e218948d072245707a"),
    "country" : "Lviv, Lviv Oblast, Ukraine"
}
{
    "_id" : ObjectId("5a36c3e218948d072245707b"),
    "country" : "Tunisia"
}

This works but it loads all the results in memory in the JavaScript Mongo Shell but would be ideal if it can be done with a single MongoDB aggregation command.

var res2 = db.coll.aggregate();

res2 = res2.toArray().map(function(doc) {
  var lastIndex = doc.locality.lastIndexOf(',');
  return doc.locality.slice(lastIndex + 2);
});

[ "Azerbaijan", "USA", "Ukraine", "Tunisia" ]

Maybe with $let , $split ?

Answer 1

You can try below aggregation query.

{"$arrayElemAt":["$$locality",-1]} to access the last element from the array.

db.coll.aggregate([{"$project":{
  "_id":0,
  "last":{
    "$let":{
      "vars":{"locality":{"$split":["$locality",","]}},
      "in":{"$arrayElemAt":["$$locality",-1]}
    }
  }
}}])