$last (aggregation accumulator)

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Definition定义

$last

Changed in version 5.0.在版本5.0中更改。

Returns the value that results from applying an expression to the last document in a group of documents. 返回将表达式应用于一组文档中的最后一个文档时产生的值。Only meaningful when documents are in a defined order.仅当文档处于定义的顺序时才有意义。

$last is available in these stages:可在以下阶段使用：

Note注意
Disambiguation消除歧义

This page describes the $last aggregation accumulator. 此页面介绍$last聚合累加器。For the $last array operator, see $last (array operator).有关$last数组运算符，请参阅$last（数组运算符）。

Syntax语法

$last syntax:语法：

{ $last: <expression> }

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

To define the document order for $last with the:要使用定义$last的文档顺序：

Note注意

Although the $sort stage passes ordered documents as input to the $group and $setWindowFields stages, those stages are not guaranteed to maintain the sort order in their own output.尽管$sort阶段将有序文档作为输入传递给$group和$setWindowFields阶段，但这些阶段不能保证在自己的输出中维护排序顺序。

When used with $setWindowFields, $last returns null for empty windows. 与$setWindowFields一起使用时，对于空窗口，$last返回null。An example empty window is a { documents: [ -1, -1 ] } documents window on the first document of a partition.空窗口的示例是分区第一个文档上的{ documents: [ -1, -1 ] }文档窗口。

Examples示例

Use in $group Stage在$group阶段中使用

Consider a sales collection with the following documents:考虑使用以下文档进行sales集合：

{ "_id" : 1, "item" : "abc", "date" : ISODate("2014-01-01T08:00:00Z"), "price" : 10, "quantity" : 2 }
{ "_id" : 2, "item" : "jkl", "date" : ISODate("2014-02-03T09:00:00Z"), "price" : 20, "quantity" : 1 }
{ "_id" : 3, "item" : "xyz", "date" : ISODate("2014-02-03T09:05:00Z"), "price" : 5, "quantity" : 5 }
{ "_id" : 4, "item" : "abc", "date" : ISODate("2014-02-15T08:00:00Z"), "price" : 10, "quantity" : 10 }
{ "_id" : 5, "item" : "xyz", "date" : ISODate("2014-02-15T09:05:00Z"), "price" : 5, "quantity" : 10 }
{ "_id" : 6, "item" : "xyz", "date" : ISODate("2014-02-15T12:05:10Z"), "price" : 5, "quantity" : 5 }
{ "_id" : 7, "item" : "xyz", "date" : ISODate("2014-02-15T14:12:12Z"), "price" : 5, "quantity" : 10 }

The following operation first sorts the documents by item and date, and then in the following $group stage, groups the now sorted documents by the item field and uses the $last accumulator to compute the last sales date for each item:以下操作首先按item和date对文档进行排序，然后在下面的$group阶段中，按项目字段对现在排序的文档进行分组，并使用$last累加器计算每个项目的最后销售日期：

db.sales.aggregate(
   [
     { $sort: { item: 1, date: 1 } },
     {
       $group:
         {
           _id: "$item",
           lastSalesDate: { $last: "$date" }
         }
     }
   ]
)

The operation returns the following results:该操作返回以下结果：

{ "_id" : "xyz", "lastSalesDate" : ISODate("2014-02-15T14:12:12Z") }
{ "_id" : "jkl", "lastSalesDate" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : "abc", "lastSalesDate" : ISODate("2014-02-15T08:00:00Z") }

Use in $setWindowFields Stage在$setWindowFields阶段中使用

New in version 5.0.在版本5.0中新增。

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 }
] )

This example uses $last in the $setWindowFields stage to output the last cake sales order type for each state:本例使用$setWindowFields阶段中的$last输出每个州的最后一个蛋糕销售订单type：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            lastOrderTypeForState: {
               $last: "$type",
               window: {
                  documents: [ "current", "unbounded" ]
               }
            }
         }
      }
   }
] )

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.有CA和WA分区。
  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate以升序（1）对每个分区中的文档进行排序，因此最早的orderDate居第一。

  • output sets the lastOrderTypeForState field to the last order type from the documents window.将lastOrderTypeForState字段设置为文档窗口中的最后一个订单type。

    The window contains documents between the current lower limit, which is the current document in the output, and the unbounded upper limit. 该窗口包含current下限（即输出中的当前文档）和unbounded上限之间的文档。This means $last returns the last order type for the documents between the current document and the end of the partition.这意味着$last返回当前文档和分区末尾之间文档的最后一个订单类型。

In this output, the last order type value for CA and WA is shown in the lastOrderTypeForState field:在此输出中，CA和WA的最后一个订单type值显示在lastOrderTypeForState字段中：

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