$last (expression operator)（表达式运算符）

Defining Document Order定义文档顺序

• To define the document order for $last in a $setWindowFields, set the sortBy field.要在$setWindowFields中定义$slast的文档顺序，请设置sortBy字段。
• To define the document order for $last in other pipeline stages, add a preceding $sort stage.要在其他管道阶段定义$last的文档顺序，请添加前面的$sort阶段。

Array Operator数组运算符

If the expression resolves to an array:如果表达式解析为数组：

• For a group of documents, as with the $group and $setWindowFields stages, $last returns the entire array from the last document. It does not traverse array elements.对于一组文档，与$group和$setWindowFields阶段一样，$last返回最后一个文档的整个数组。它不遍历数组元素。
• For an individual document, as with the $addFields stage, $last returns the last element of the array.对于单个文档，与$addFields阶段一样，$last返回数组的最后一个元素。

Missing Values缺失值

Documents in a group may be missing fields or may have fields with missing values.组中的文档可能缺少字段，或者可能有缺少值的字段。

• If there are no documents from the prior pipeline stage, the $group stage returns nothing.如果没有来自前一个管道阶段的文档，则$group阶段将不返回任何内容。
• If the field that the $last accumulator is processing is missing, $last returns null.如果$last累加器正在处理的字段丢失，$last将返回null。
• When used with $setWindowFields, $last returns null for empty windows. For example, when you have a { documents: [ -1, -1] } documents window on the first document of a partition.当与$setWindowFields一起使用时，$last对于空窗口返回null。例如，当您在分区的第一个文档上有一个{ documents: [ -1, -1] } documents窗口时。

For more information, see the Missing Data example later in this topic.有关更多信息，请参阅本主题后面的缺少数据示例。

Use in $group Stage在$group Stage中使用

Consider a sales collection with the following documents:考虑一个包含以下文件的sales集合：

db.sales.insertMany( [
   { _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 $last accumulator to compute the last sales date for each item:以下操作首先按item和date对文档进行排序，然后在接下来的$group阶段，按item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

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):创建一个cakeSales集合，其中包含加利福尼亚州（CA）和华盛顿州（WA）的蛋糕销售：

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 $setWindowFields stage to output the last cake sales order type for each state:此示例在$setWindowFields阶段使用$last输出每个state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. There are partitions for CA and WA.按state对集合中的文档进行分区。CA和WA有分区。
• sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate升序（1）对每个分区中的文档进行排序，因此最早的orderDate位居前列。

• output sets the lastOrderTypeForState field to the last order type from the documents window.output将lastOrderTypeForState字段设置为documents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window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的文档的最后一个订单type。

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" }