$firstN (expression operator)（表达式运算符）

Null and Missing Valuesnull值和缺失值

• $firstN does not filter out null values.不筛选null值。
• $firstN converts missing values to null.将缺失的值转换为null。

Consider the following aggregation that returns the first five documents from a group:考虑以下聚合，它返回组中的前五个文档：

db.aggregate( [
   {
      $documents: [
         { playerId: "PlayerA", gameId: "G1", score: 1 },
         { playerId: "PlayerB", gameId: "G1", score: 2 },
         { playerId: "PlayerC", gameId: "G1", score: 3 },
         { playerId: "PlayerD", gameId: "G1"},
         { playerId: "PlayerE", gameId: "G1", score: null }
      ]
   },
   {
      $group:
      {
         _id: "$gameId",
         firstFiveScores:
            {
               $firstN:
                  {
                     input: "$score",
                     n: 5
                  }
            }
      }
   }
] )

In this example:在这个例子中：

• $documents creates the literal documents that contain player scores.创建包含玩家分数的文字文档。
• $group groups the documents by gameId. This example has only one gameId, G1.按gameId对文档进行分组。这个例子只有一个gameId G1。
• PlayerD has a missing score and PlayerE has a null score. 缺少score，而PlayerE的score为空。These values are both considered as null.这些值都被视为null。
• The firstFiveScores field is specified using input : "$score" and returned as an array.firstFiveScores字段使用input : "$score"指定，并作为数组返回。
• Since there is no sort criteria the first 5 score fields are returned.由于没有排序标准，因此返回前5个score字段。

[
   {
      _id: 'G1',
      firstFiveScores: [ 1, 2, 3, null, null ]
   }
]

Comparison of $firstN and $topN Accumulators$firstN和$topN累加器的比较

Both $firstN and $topN accumulators can accomplish similar results.$firstN和$topN累加器都可以实现类似的结果。

In general:一般来说：

• If the documents coming into $group are already ordered, you should use $firstN.如果进入$group的文档已经排序，则应使用$firstN。
• If you're sorting and selecting the top n elements then you can use $topN to accomplish both tasks with one accumulator.如果你正在排序和选择前n个元素，那么你可以使用$topN用一个累加器来完成这两个任务。
• $firstN can be used as an aggregation expression, $topN cannot.$firstN可以用作聚合表达式，$topN不能。

Window Function and Aggregation Expression Support窗口函数和聚合表达式支持

$firstN is supported as an aggregation expression.支持作为聚合表达式。

$firstN is supported as a window operator.支持作为窗口运算符。

Find the First Three Player Scores for a Single Game查找单场比赛的前三名玩家得分

You can use the $firstN accumulator to find the first three scores in a single game.您可以使用$firstN累加器来查找单局中的前三个分数。

db.gamescores.aggregate( [
   {
      $match : { gameId : "G1" }
   },
   {
      $group:
         {
            _id: "$gameId",
            firstThreeScores:
               {
                  $firstN:
                  {
                     input: ["$playerId", "$score"],
                     n:3
                  }
               }
         }
   }
] )

The example pipeline:示例管道：

• Uses $match to filter the results on a single gameId. In this case, G1.使用$match筛选单个gameId的结果。在这种情况下，G1。
• Uses $group to group the results by gameId. In this case, G1.使用$group按gameId对结果进行分组。在这种情况下，G1。
• Specifies the fields that are input for $firstN with input : ["$playerId"," $score"].通过input : ["$playerId"," $score"]指定为$firstN输入的字段。
• Uses $firstN to return the first three documents for the G1 game with n : 3.使用$firstN配合n:3返回G1游戏的前三个文档。

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

[
   {
      _id: 'G1',
      firstThreeScores: [ [ 'PlayerA', 31 ], [ 'PlayerB', 33 ], [ 'PlayerC', 99 ] ]
   }
]

Finding the First Three Player Scores Across Multiple Games在多个游戏中找到前三名玩家的分数

You can use the $firstN accumulator to find the first n input fields in each game.您可以使用$firstN累加器来查找每个游戏中的前n个输入字段。

db.gamescores.aggregate( [
   {
      $group:
      {
      _id: "$gameId", playerId:
         {
            $firstN:
               {
                  input: [ "$playerId","$score" ],
                  n: 3
               }
         }
      }
   }
] )

The example pipeline:示例管道：

• Uses $group to group the results by gameId.使用$group按gameId对结果进行分组。
• Uses $firstN to return the first three documents for each game with n: 3.使用$firstN配合n:3返回每个游戏的前三个文档。
• Specifies the fields that are input for $firstN with input : ["$playerId", "$score"].通过input : ["$playerId", "$score"]指定为$firstN输入的字段。

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

[
   {
      _id: 'G1',
      playerId: [ [ 'PlayerA', 31 ], [ 'PlayerB', 33 ], [ 'PlayerC', 99 ] ]
   },
   {
      _id: 'G2',
      playerId: [ [ 'PlayerA', 10 ], [ 'PlayerB', 14 ], [ 'PlayerC', 66 ] ]
   }
]

Using $sort With $firstN使用$sort和$firstN

Using a $sort stage earlier in the pipeline can influence the results of the $firstN accumulator.在管道的早期使用$sort阶段会影响$firstN累加器的结果。

In this example:在这个例子中：

• {$sort : { score : -1 } } sorts the highest scores to the back of each group.将最高分数排序到每组的后面。
• firstN returns the three highest scores from front of each group.从每组前面返回三个最高分数。

db.gamescores.aggregate( [
      { $sort : { score : -1 } },
      {

         $group:
         { _id: "$gameId", playerId:
            {
               $firstN:
                  {
                     input: [ "$playerId","$score" ],
                     n: 3
                  }
            }
         }
      }
   ] )

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

[
   {
      _id: 'G2',
      playerId: [ [ 'PlayerD', 80 ], [ 'PlayerC', 66 ], [ 'PlayerB', 14 ] ]
   },
   {
      _id: 'G1',
      playerId: [ [ 'PlayerC', 99 ], [ 'PlayerB', 33 ], [ 'PlayerA', 31 ] ]
   }
]

Computing n Based on the Group Key for $group基于$Group的组键计算n

You can also assign the value of n dynamically. In this example, the $cond expression is used on the gameId field.您还可以动态地分配n的值。在这个例子中，$cond表达式用于gameId字段。

db.gamescores.aggregate([
   {
      $group:
      {
         _id: {"gameId": "$gameId"},
         gamescores:
            {
               $firstN:
                  {
                     input: "$score",
                     n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } }
                  }
            }
      }
   }
] )

The example pipeline:示例管道：

• Uses $group to group the results by gameId.使用$group按gameId对结果进行分组。
• Specifies the fields that input for $firstN with input : "$score".使用input : "$score"为$firstN指定输入的字段。
• If the gameId is G2 then n is 1, otherwise n is 3.如果gameId为G2，则n为1，否则n为3。

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

[
   { _id: { gameId: 'G1' }, gamescores: [ 31, 33, 99 ] },
   { _id: { gameId: 'G2' }, gamescores: [ 10 ] }
]

Using $firstN as an Aggregation Expression使用$firstN作为聚合表达式

You can also use $firstN as an aggregation expression.您还可以将$firstN用作聚合表达式。

In this example:在这个例子中：

• $documents creates the literal document that contains an array of values.创建包含值数组的文本文档。
• $project is used to return the output of $firstN.用于返回$firstN的输出。
• _id is omited from the output with _id : 0.使用_id : 0从输出中省略_id。
• $firstN uses the input array of [10, 20, 30, 40].使用[10,20,30,40]的输入数组。
• The first three elements of the array are returned for the input document.为输入文档返回数组的前三个元素。

db.aggregate( [
   {
      $documents: [
         { array: [10, 20, 30, 40] } ]
   },
   { $project: {
      firstThreeElements:{
                           $firstN:
                           {
                              input: "$array",
                              n: 3
                           }
                        }
               }
   }
] )

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

[
   { firstThreeElements: [ 10, 20, 30 ] }
]