Mongo addFields从嵌套数组中获取值

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

db.students.aggregate([
  {
    $addFields: {
      mathScore: {
        $arrayElemAt: [
          {
            $filter: {
              input: "$grades",
              as: "grade",
              cond: {
                $eq: ["$$grade.subject", "Math"]
              }
            }
          },
          0
        ]
      }
    }
  }
])

{
  "_id": 1,
  "name": "John",
  "grades": [
    {
      "subject": "Math",
      "score": 90
    },
    {
      "subject": "English",
      "score": 85
    }
  ],
  "mathScore": {
    "subject": "Math",
    "score": 90
  }
}