$first (aggregation accumulator)

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Definition定义

$first

Changed in version 5.0.在版本5.0中更改。

Returns the value that results from applying an expression to the first document in a group of documents. 返回将表达式应用于一组文档中的第一个文档所产生的值。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。

$first is available in these stages:在以下阶段可用：

Note注意
Disambiguation消除歧义

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

Syntax语法

$first syntax:语法：

{ $first: <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 $first with the:要定义$first的文档顺序，请使用：

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, $first returns null for empty windows. 与$setWindowFields一起使用时，对于空窗口，$first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。

Missing Values缺少值

The 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，也可能包含缺少值的字段。

  • If there are no documents from the prior pipeline stage, the $group stage returns nothing.如果没有来自上一管道阶段的文档，$group阶段将不返回任何内容。
  • If the field that the $first accumulator is processing is missing, $first returns null.如果缺少$first累加器正在处理的字段，$first将返回null。

See the missing data example.请参阅缺失数据示例。

Examples示例

Use in $group Stage在$group阶段中使用

Create the sales collection:创建sales集合：

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

Grouping the documents by the item field, the following operation uses the $first accumulator to compute the first sales date for each item:以下操作按item字段对文档进行分组，使用$first累加器计算每个项目的第一个销售日期：

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

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

[
   { _id: 'jkl', firstSale: ISODate("2014-02-03T09:00:00.000Z") },
   { _id: 'xyz', firstSale: ISODate("2014-02-03T09:05:00.000Z") },
   { _id: 'abc', firstSale: ISODate("2014-01-01T08:00:00.000Z") }
]

Missing Data缺少数据

Some documents in the badData collection are missing fields, other documents are missing values.badData集合中的某些文档缺少字段，其他文档缺少值。

Create the badData collection:创建badData集合：

db.badData.insertMany( [
   { "_id": 1, "price": 6, "quantity": 6 },
   { "_id": 2, "item": "album", "price": 5 , "quantity": 5  },
   { "_id": 7, "item": "tape", "price": 6, "quantity": 6 },
   { "_id": 8, "price": 5, "quantity": 5  },
   { "_id": 9, "item": "album", "price": 3, "quantity": '' },
   { "_id": 10, "item": "tape", "price": 3, "quantity":  4 },
   { "_id": 12, "item": "cd", "price": 7  }
] )

Query the badData collection, grouping the output on the item field:查询badData集合，将item字段上的输出分组：

db.badData.aggregate( [
   { $sort: { item: 1, price: 1 } },
   { $group:
      {
         _id: "$item",
         inStock: { $first: "$quantity" }
      }
   }
] )

The $sort stage orders the documents and passes them to the $group stage.$sort阶段对文档进行排序并将其传递给$group阶段。

[
  { _id: null, inStock: 5 },
  { _id: 'album', inStock: '' },
  { _id: 'cd', inStock: null },
  { _id: 'tape', inStock: 4 }
]

$first selects the first document from each output group:从每个输出组中选择第一个文档：

  • The _id: null group is included.包含_id:null组。
  • When the accumulator field, $quantity in this example, is missing, $first returns null.当缺少累加器字段（本例中为$quantity）时，$first返回null。

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系列：

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 $first in the $setWindowFields stage to output the first cake sales order type for each state:本例使用$setWindowFields阶段中的$first为每个state输出第一个蛋糕销售订单type：

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

In the example:在该示例中：

  • 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.有用于CA和WA的分区。
  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate按升序（1）对每个分区中的文档进行排序，因此最早的orderDate是第一个。

  • output sets the firstOrderTypeForState field to the first order type from the documents window.将文档窗口中的firstOrderTypeForState字段设置为第一个订单type。

    The window contains documents between an unbounded lower limit and the current document in the output. 该窗口包含输出中处于unbounded下限和current文档之间的文档。This means $first returns the first order type for the documents between the beginning of the partition and the current document.这意味着$first返回分区开头和当前文档之间的文档的第一顺序type。

In this output, the first order type value for CA and WA is shown in the firstOrderTypeForState field:在此输出中，CA和WA的第一个订单type值显示在firstOrderTypeForState字段中：

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