如何在聚合管道的$group阶段将数组转换为对象？

db.collection.aggregate([
  {
    $group: {
      _id: "$groupField",
      newArray: { $push: "$arrayField" }
    }
  },
  {
    $project: {
      convertedObject: {
        $reduce: {
          input: "$newArray",
          initialValue: {},
          in: {
            $concatArrays: [
              "$$value",
              { $arrayToObject: [[{ k: "$$this.key", v: "$$this.value" }]] }
            ]
          }
        }
      }
    }
  },
  {
    $replaceRoot: {
      newRoot: "$convertedObject"
    }
  }
])