1.按region和business分组，并将country推入countries数组。
1.按region分组并推送键值对（k：business，v：countries）到business阵列中。
1.设置business字段以将每个键值对转换为对象并匹配所需的输出。

db.collection.aggregate([
  {
    $group: {
      _id: {
        region: "$region",
        business: "$business"
      },
      countries: {
        $push: "$country"
      }
    }
  },
  {
    $group: {
      _id: "$_id.region",
      business: {
        $push: {
          k: "$_id.business",
          v: "$countries"
        }
      }
    }
  },
  {
    $set: {
      business: {
        $map: {
          input: "$business",
          in: {
            $arrayToObject: [
              [
                "$$this"
              ]
            ]
          }
        }
      }
    }
  }
])

Demo @ Mongo Playground