Filter by last element in nested array in MongoDb

Question

let's assume I have this object in mongo:

    {
      "_id": "1",
      "Test" {
        "Array": [ new NumberInt("10") ]
      }
    }

Now I would like to use this filter to check if last element in an array satisfy my condition:

    { "Test.Array.-1": { "$gte": new NumberInt("10") } }

But it doesn't work as I expected it to. Is it impossible to filter by last element in an array or do I do something wrong here?

ray · Accepted Answer · 2022-12-08 14:39:38Z

3

Chain up $gte with $arrayElemAt in a $expr

db.collection.find({
  $expr: {
    $gte: [
      {
        $arrayElemAt: [
          "$Test.Array",
          -1
        ]
      },
      10
    ]
  }
})

Mongo Playground

answered Dec 8, 2022 at 14:39

ray

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2 Comments

nimrod serok Over a year ago

You can also use $last, like this, which is the same, just a bit cleaner (in my opinion)

ray Over a year ago

@nimrodserok Thanks for enlightening me with the usage of $last. I thought we are only allowed to use it in a $group lol

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