$first (aggregation)

Defining Document Order定义文档顺序

• To define the document order for $first in a $setWindowFields, set the sortBy field.要在$setWindowFields中定义$first的文档顺序，请设置sortBy字段。
• To define the document order for $first in other pipeline stages, add a preceding $sort stage.要在其他管道阶段中为$first定义文档顺序，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, $first returns the entire array from the first document. 对于一组文档，$first从第一个文档返回整个数组。It does not traverse array elements. 它不遍历数组元素。This is the case with stages like $group and $setWindowFields.像$group和$setWindowFields这样的阶段就是这样。
• For an individual document, $first returns the first element of the array. 对于单个文档，$first返回数组的第一个元素。This is the case with stages like $addFields.像$addFields这样的阶段就是这样。

Missing Values缺少值

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，也可能具有缺少值的字段。

• If there are no documents from the prior pipeline stage, the $group stage returns nothing.如果没有来自上一个管道阶段的文档，$group阶段将不返回任何内容。
• If the field that the $first accumulator is processing is missing, $first returns null.如果$first累加器正在处理的字段丢失，$first将返回null。
• When used with $setWindowFields, $first returns null for empty windows. For example, when you have a { documents:[ -1, -1] } documents window on the first document of a partition.与$setWindowFields一起使用时，$first会为空窗口返回null。例如，当分区的第一个文档上有一个documents窗口{ documents:[ -1, -1] }时。

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

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 $first accumulator to return the first sales date for each item:根据item字段对文档进行分组，以下操作使用$first累加器返回每个物料的第一个销售日期：

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集合中的一些文档缺少字段，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code>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，并将它们传递到$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.包括_id:null组。
• When the accumulator field, $quantity in this example, is missing, $first returns null.当累加器字段（本例中为$quantity）丢失时，$first将返回null。

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为每个状态输出第一个蛋糕销售订单类型：

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

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firstOrderTypeForState字段设置为documents窗口中的第一个订单type。

  The window contains documents between an unbounded lower limit and the current document in the output. 该window包含在unbounded下限和输出中的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返回分区开始和当前文档之间的文档的第一个订单类型。

In this output, the first order type value for CA and WA is shown in the firstOrderTypeForState field:在该输出中，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" }