$shift (aggregation)

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Definition定义

New in version 5.0.在版本5.0中新增。

$shift

Returns the value from an expression applied to a document in a specified position relative to the current document in the $setWindowFields stage partition.返回应用于文档的表达式中的值，该文档位于相对于$setWindowFields阶段分区中当前文档的指定位置。

The $setWindowFields stage sortBy field value determines the document order. $setWindowFields阶段sortBy字段值决定文档顺序。For more information on how MongoDB compares fields with different types, see BSON comparison order.有关MongoDB如何比较不同类型字段的更多信息，请参阅BSON比较顺序。

$shift is only available in the $setWindowFields stage.仅在$setWindowFields阶段可用。

$shift syntax:语法：

{
   $shift: {
      output: <output expression>,
      by: <integer>,
      default: <default expression>
   }
}

$shift takes a document with these fields:获取包含以下字段的文档：

Field字段Description描述
output

Specifies an expression to evaluate and return in the output.指定要在输出中计算和返回的表达式。

by

Specifies an integer with a numeric document position relative to the current document in the output.指定一个integer，其中包含相对于输出中当前文档的数字文档位置。

For example:例如：

  • 1 specifies the document position after the current document.指定当前文档之后的文档位置。
  • -1 specifies the document position before the current document.指定当前文档之前的文档位置。
  • -2 specifies the document position that is two positions before the current document.指定文档位置，即当前文档之前的两个位置。
default

Specifies an optional default expression to evaluate if the document position is outside of the implicit $setWindowFields stage window. 指定可选的默认表达式，以评估文档位置是否在隐式$setWindowFields阶段窗口之外。The implicit window contains all the documents in the partition.隐式窗口包含分区中的所有文档。

The default expression must evaluate to a constant value.default表达式的计算结果必须为常量值。

If you do not specify a default expression, $shift returns null for documents whose positions are outside of the implicit $setWindowFields stage window.如果不指定default表达式，对于位置在隐式$setWindowFields阶段窗口之外的文档，$shift将返回null。

Behavior行为

$shift returns an error if you specify a window in the $setWindowFields stage.如果在$setWindowFields阶段指定窗口，则$shift返回错误。

Examples示例

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):创建包含加利福尼亚州（CA）和华盛顿州（WA）蛋糕销售的cakeSales集合：

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

The cakeSales collection is used in the following examples.以下示例中使用了cakeSales集合。

Shift Using a Positive Integer使用正整数移位

This example uses $shift in the $setWindowFields stage to output the quantity of the cake sales from each document following the current document for each state:本例使用$setWindowFields阶段中的$shift输出每个state当前文档后面的每个文档中的蛋糕销售quantity：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { quantity: -1 },
         output: {
            shiftQuantityForState: {
               $shift: {
                  output: "$quantity",
                  by: 1,
                  default: "Not available"
               }
            }
         }
      }
   }
] )

In the example:在示例中：

  • partitionBy: "$state" partitions the documents in the collection by state. 按state对集合中的文档进行分区。There are partitions for CA and WA.CA和WA有分区。
  • sortBy: { quantity: -1 } sorts the documents in each partition by quantity in descending order (-1), so the highest quantity is first.按quantity按降序（-1）对每个分区中的文档进行排序，因此quantity最高的是第一个。

  • output after sortBy:sortBy后的output：

    • Sets the shiftQuantityForState field to the quantity value from the documents in each state.将shiftQuantityForState字段设置为每个state的文档的quantity值。

    • Uses $shift to return the quantity value from the document that follows the current document in the output.使用$shift从输出中当前文档之后的文档中返回quantity值。

      • The document position is specified using the $shiftby integer set to 1.文档位置是使用$shift将by设置为整数1指定的。
      • For documents outside of the implicit window, $shift returns "Not available", which is specified using the default expression.对于隐式窗口之外的文档，$shift返回"Not available"，这是使用default表达式指定的。

In this example output, the shifted quantity value is shown in the shiftQuantityForState field for each returned document:在这个示例输出中，每个返回的文档的shiftQuantityForState字段中都显示了移位的quantity值：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "shiftQuantityForState" : 145 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "shiftQuantityForState" : 120 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "shiftQuantityForState" : "Not available" }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "shiftQuantityForState" : 134 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "shiftQuantityForState" : 104 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "shiftQuantityForState" : "Not available" }

Shift Using a Negative Integer使用负整数移位

This example uses $shift in the $setWindowFields stage to output the quantity of the cake sales from each document before the current document for each state:本例使用$setWindowFields阶段中的$shift输出每个州state当前文档之前每个文档的蛋糕销售quantity：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { quantity: -1 },
         output: {
            shiftQuantityForState: {
               $shift: {
                  output: "$quantity",
                  by: -1,
                  default: "Not available"
               }
            }
         }
      }
   }
] )

In the example:在示例中：

  • partitionBy: "$state" partitions the documents in the collection by state. 按state对集合中的文档进行分区。There are partitions for CA and WA.CA和WA有分区。
  • sortBy: { quantity: -1 } sorts the documents in each partition by quantity in descending order (-1), so the highest quantity is first.按quantity按降序（-1）对每个分区中的文档进行排序，因此quantity最高的是第一个。

  • output after sortBy:sortBy后的output：

    • Sets the shiftQuantityForState field to the quantity value from the documents in each state.将shiftQuantityForState字段设置为每个state的文档的quantity值。

    • Uses $shift to return the quantity value from the document before the current document in the output.使用$shift从输出中当前文档之前的文档中返回quantity值。

      • The document position is specified using the $shiftby integer set to -1.文档位置是使用$shift将by设置为整数-1指定的，。
      • For documents outside of the implicit window, $shift returns "Not available", which is specified using the default expression.对于隐式窗口之外的文档，$shift返回"Not available"，这是使用default表达式指定的。

In this example output, the shifted quantity value is shown in the shiftQuantityForState field for each returned document:在这个示例输出中，每个返回的文档的shiftQuantityForState字段中都显示了移位的quantity值：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "shiftQuantityForState" : "Not available" }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "shiftQuantityForState" : 162 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "shiftQuantityForState" : 145 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "shiftQuantityForState" : "Not available" }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "shiftQuantityForState" : 140 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "shiftQuantityForState" : 134 }
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