MongoDB :文档中两个数组的聚合值(映射)

{
  "_id": 1,
  "name": "Alice",
  "scores": [
    { "subject": "Math", "score": 90 },
    { "subject": "English", "score": 85 }
  ]
}

{
  "_id": 1,
  "name": "Alice",
  "grades": [
    { "subject": "Math", "grade": "A" },
    { "subject": "English", "grade": "B" }
  ]
}

db.students.aggregate([
  {
    $project: {
      _id: 1,
      name: 1,
      grades: {
        $map: {
          input: "$scores",
          as: "score",
          in: {
            subject: "$$score.subject",
            grade: {
              $cond: [
                { $gte: ["$$score.score", 90] }, "A",
                { $gte: ["$$score.score", 80] }, "B",
                { $gte: ["$$score.score", 70] }, "C",
                "D"
              ]
            }
          }
        }
      }
    }
  }
])