$locf (aggregation)

Comparison of $fill and $locf$fill和$locf的比较

To fill missing field values based on the last observed value in a sequence, you can use:要根据序列中最后一个观察到的值填充缺失的字段值，可以使用：

• The $fill stage with { method: "locf" }.带有{ method: "locf" }的$fill阶段。

  When you use the $fill stage, the field you specify in the output is the same field used as the source data. See Fill Missing Field Values Based on the Last Observed Value.使用$fill阶段时，在输出中指定的字段与源数据使用的字段相同。请参见根据上次观测值填充缺少的字段值。

• The $locf operator inside of a $setWindowFields stage.$setWindowFields阶段内的$locf运算符。

  When you use the $locf operator, you can set values for a different field than the field used as the source data. See Use Multiple Fill Methods in a Single Stage.使用$locf运算符时，可以为不同于用作源数据的字段的字段设置值。请参见在单个阶段中使用多种填充方法。

Fill Missing Values with the Last Observed Value用上次观测值填充缺失值

The following example uses the $locf operator to set missing fields to the value from the last-observed non-null value:以下示例使用$locf运算符将缺少的字段设置为上次观察到的非null值中的值：

db.stock.aggregate( [
   {
      $setWindowFields: {
        sortBy: { time: 1 },
        output: {
           price: { $locf: "$price" }
        }
      }
    }
] )

In the example:在示例中：

• sortBy: { time: 1 } sorts the documents in each partition by time in ascending order (1), so the earliest time is first.按time升序（1）对每个分区中的文档进行排序，因此最早的time是第一个。
• For documents where the price field is missing, the $locf operator sets the price to the last-observed value in the sequence.对于缺少price字段的文档，$locf运算符将price设置为序列中最后一个观察到的值。

Example output:示例输出：

[
  {
    _id: ObjectId("62169b65394d47411658b5f5"),
    time: ISODate("2021-03-08T09:00:00.000Z"),
    price: 500
  },
  {
    _id: ObjectId("62169b65394d47411658b5f6"),
    time: ISODate("2021-03-08T10:00:00.000Z"),
    price: 500
  },
  {
    _id: ObjectId("62169b65394d47411658b5f7"),
    time: ISODate("2021-03-08T11:00:00.000Z"),
    price: 515
  },
  {
    _id: ObjectId("62169b65394d47411658b5f8"),
    time: ISODate("2021-03-08T12:00:00.000Z"),
    price: 515
  },
  {
    _id: ObjectId("62169b65394d47411658b5f9"),
    time: ISODate("2021-03-08T13:00:00.000Z"),
    price: 515
  },
  {
    _id: ObjectId("62169b65394d47411658b5fa"),
    time: ISODate("2021-03-08T14:00:00.000Z"),
    price: 485
  }
]

Use Multiple Fill Methods in a Single Stage在单个阶段中使用多种填充方法

When you use the $setWindowFields stage to fill missing values, you can set values for a different field than the field you fill from. 当您使用$setWindowFields阶段来填充缺失的值时，您可以为不同于从中填充的字段的字段设置值。As a result, you can use multiple fill methods in a single $setWindowFields stage and output the results in distinct fields.因此，您可以在单个$setWindowFields阶段中使用多个填充方法，并在不同的字段中输出结果。

The following pipeline populates missing price fields using linear interpolation and the last-observation-carried-forward method:以下管道使用线性插值和最后一次观测结转方法填充缺失的price字段：

db.stock.aggregate( [
   {
      $setWindowFields:
         {
            sortBy: { time: 1 },
            output:
               {
                  linearFillPrice: { $linearFill: "$price" },
                  locfPrice: { $locf: "$price" }
               }
         }
   }
] )

In the example:在示例中：

• sortBy: { time: 1 } sorts the documents by the time field in ascending order, from earliest to latest.按照time字段从最早到最晚的升序对文档进行排序。
• output specifies:指定：
  • linearFillPrice as a target field to be filled.作为要填充的目标字段。
    • { $linearFill: "$price" } is the value for the linearFillPrice field. $linearFill fills missing price values using linear interpolation based on the surrounding price values in the sequence.是linearFillPrice字段的值。$linearFill使用基于序列中周围price值的线性插值来填充缺失的price值。
  • locfPrice as a target field to be filled.作为要填充的目标字段。
    • { $locf: "$price" } is the value for the locfPrice field. 是locfPrice字段的值。locf stands for last observation carried forward. 代表最后一次观测。$locf fills missing price values with the value from the previous document in the sequence.用序列中上一个文档的值填充缺少的price值。

Example output:输出示例：

[
  {
    _id: ObjectId("620ad555394d47411658b5ef"),
    time: ISODate("2021-03-08T09:00:00.000Z"),
    price: 500,
    linearFillPrice: 500,
    locfPrice: 500
  },
  {
    _id: ObjectId("620ad555394d47411658b5f0"),
    time: ISODate("2021-03-08T10:00:00.000Z"),
    linearFillPrice: 507.5,
    locfPrice: 500
  },
  {
    _id: ObjectId("620ad555394d47411658b5f1"),
    time: ISODate("2021-03-08T11:00:00.000Z"),
    price: 515,
    linearFillPrice: 515,
    locfPrice: 515
  },
  {
    _id: ObjectId("620ad555394d47411658b5f2"),
    time: ISODate("2021-03-08T12:00:00.000Z"),
    linearFillPrice: 505,
    locfPrice: 515
  },
  {
    _id: ObjectId("620ad555394d47411658b5f3"),
    time: ISODate("2021-03-08T13:00:00.000Z"),
    linearFillPrice: 495,
    locfPrice: 515
  },
  {
    _id: ObjectId("620ad555394d47411658b5f4"),
    time: ISODate("2021-03-08T14:00:00.000Z"),
    price: 485,
    linearFillPrice: 485,
    locfPrice: 485
  }
]