How to transform an array field into a value equaling its maximum?

Question

{
   name: "use_name",
   grades: [
     {class: "math": grade: 100},
     {class: "english": grade: 90}
   ]
}

How do I write an aggregation pipeline to output:

{
   name: "use_name",
   grades: {class: "math": grade: 100},
   
}

The grades field has been reduced to the element where its grade property is the maximum of all elements.

The requirements, the aggregation pipeline cannot have $unwind or $group because it cannot have a stage where the stage needs to receive all incoming documents before outputting to the next stage, potentially exceeding the 100mb limit. And it must be fast.

Answer 1

I think this one is faster:

db.collection.aggregate([
  {
    $set: {
      grades: {
        $first: {
          $sortArray: {
            input: "$grades",
            sortBy: { grade: -1 }
          }
        }
      }
    }
  }
])

Mongo Playground

or this one:

db.collection.aggregate([
  {
    $set: {
      grades: {
        $filter: {
          input: "$grades",
          cond: { $eq: [ "$$this.grade", { $max: "$grades.grade" } ] }
        }
      }
    }
  }
])