Node.js 如何获取文件的中间值

从MongoDB版本5.0开始，一种 Select 是使用$setWindowFields:

db.collection.aggregate([
  {$setWindowFields: {
      sortBy: {points: -1},
      output: {
        neighbors: {
          $push: "$$ROOT",
          window: {documents: [-N, N]}
        }
      }
  }},
  {$match: {_id: _id}},
  {$project: {neighbors: 1, _id: 0}},
  {$unwind: {path: "$neighbors", includeArrayIndex: "index"}},
  {$project: {
      _id: "$neighbors._id",
      points: "$neighbors.points",
      index: 1
  }}
])

看看它在playground example号公路上是如何工作的

• 如果文档很重，或者N是一个大数字，您可以只按下_id，然后在$match之后使用$lookup

EDIT:如果您想要根据整个排序进行索引:

1. 在$setWindowFields上添加另一个输出字段以设置索引
2. 使用$reduce设置为我们使用的所有项目编制索引

db.collection.aggregate([
  {$setWindowFields: {
      sortBy: {points: -1},
      output: {
        neighbors: {
          $push: "$$ROOT",
          window: {documents: [-N, N]}
        },
        index: {
          $sum: 1,
          window: {documents: ["unbounded", "current"]}
        }
      }
  }},
  {$match: {_id: _id}},
  {$project: {
      neighbors: {$reduce: {
          input: "$neighbors",
          initialValue: [],
          in: {$concatArrays: [
              "$$value",
              [{$mergeObjects: [
                    "$$this",
                    {index: {$add: [{$subtract: [{$size: "$$value"}, N + 1]}, "$index"]}}
              ]}]
          ]}
      }},
      _id: 0
  }},
  {$unwind: "$neighbors"},
  {$replaceRoot: {newRoot: "$neighbors"}}
])

看看它在playground example号公路上是如何工作的