首先是将两个数组映射为一个数组。

其次是过滤单个数组,使其只包含HIGH(/LOW),并将过滤后的数组减少为单个sum元素。

[{$project: {
  "Quality": {
    $map: {
      input: {$range: [0, {$size: "$Quality"}]},
      as: "idx",
      in: {
        "Quality": { $arrayElemAt: ["$Quality", "$$idx"] },
        "Pages": { $arrayElemAt: ["$Pages", "$$idx"] }
      }
    }
  }
}}, {$project: {
  "HIGH": {
    $reduce: {
      input: {
        $filter: {
          input: "$Quality",
          as: "x",
          cond: {$eq: ["$$x.Quality", "HIGH"]}
        }
      },
      initialValue: 0,
      in: {
        $add: ["$$value", "$$this.Pages"]
      }
    }
  },
  "LOW": {
    $reduce: {
      input: {
        $filter: {
          input: "$Quality",
          as: "x",
          cond: {$eq: ["$$x.Quality", "LOW"]}
        }
      },
      initialValue: 0,
      in: {
        $add: ["$$value", "$$this.Pages"]
      }
    }
  }
}}]

这正是你所要求的:

{
  _id: 1
  HIGH:27
  LOW:22
}