按逗号分隔字符串的长度聚合的MongoDB查询

db.collection.aggregate([
  {
    $project: {
      tagLength: { $size: { $split: ["$tags", ","] } }
    }
  },
  {
    $group: {
      _id: "$tagLength",
      count: { $sum: 1 }
    }
  },
  {
    $sort: { _id: 1 }
  }
])

db.collection.aggregate([
  {
    $addFields: {
      tagsArray: {
        $cond: {
          if: { $isArray: "$tags" },
          then: "$tags",
          else: { $split: ["$tags", ","] }
        }
      }
    }
  },
  {
    $project: {
      tagLength: { $size: "$tagsArray" }
    }
  },
  {
    $group: {
      _id: "$tagLength",
      count: { $sum: 1 }
    }
  },
  {
    $sort: { _id: 1 }
  }
])