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$slice

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$slice

The $slice modifier limits the number of array elements during a $push operation. To project, or return, a specified number of array elements from a read operation, see the $slice projection operator instead.$slice修饰符在$push操作期间限制数组元素的数量。要从读取操作投影或返回指定数量的数组元素，请参阅$slice投影运算符。

To use the $slice modifier, it must appear with the $each modifier. 若要使用$slice修饰符，它必须与$each修饰符一起出现。You can pass an empty array [] to the $each modifier such that only the $slice modifier has an effect.您可以将一个空数组[]传递给$each修饰符，这样只有$slice修饰符才有效果。

{
  $push: {
     <field>: {
       $each: [ <value1>, <value2>, ... ],
       $slice: <num>
     }
  }
}

The <num> can be:<num>可以是：

Value值Description描述
Zero零To update the array <field> to an empty array.将数组<field>更新为空数组。
Negative负数To update the array <field> to contain only the last <num> elements.将数组<field>更新为只包含最后的<num>元素。
Positive正数To update the array <field> contain only the first <num> elements.要更新数组，<field>只包含前<num>个元素。

Behavior行为

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. 从MongoDB 5.0开始，update运算符按照字典顺序处理具有基于字符串的名称的文档字段。Fields with numeric names are processed in numeric order. 具有数字名称的字段按数字顺序处理。See Update Operators Behavior for details.有关详细信息，请参阅更新运算符行为。

The order in which the modifiers appear is immaterial. 修饰语的出现顺序无关紧要。Previous versions required the $each modifier to appear as the first modifier if used in conjunction with $slice. For a list of modifiers available for $push, see Modifiers.以前的版本要求$each修饰符在与$slice一起使用时显示为第一个修饰符。有关$push可用的修改器列表，请参阅修饰符。

Trying to use the $slice modifier without the $each modifier results in an error.尝试在没有$slice修饰符的情况下使用$each修饰符会导致错误。

Examples实例

Slice from the End of the Array从数组末端分片

A collection students contains the following document:students集合包含以下文档：

{ "_id" : 1, "scores" : [ 40, 50, 60 ] }

The following operation adds new elements to the scores array, and then uses the $slice modifier to trim the array to the last five elements:以下操作将新元素添加到scores数组中，然后使用$slice修饰符将数组修剪为最后五个元素：

db.students.updateOne(
   { _id: 1 },
   {
     $push: {
       scores: {
         $each: [ 80, 78, 86 ],
         $slice: -5
       }
     }
   }
)

The result of the operation is slice the elements of the updated scores array to the last five elements:操作的结果是将更新的scores数组的元素分片为最后五个元素：

{ "_id" : 1, "scores" : [  50,  60,  80,  78,  86 ] }

Slice from the Front of the Array数组前面的分片

A collection students contains the following document:students集合包含以下文档：

{ "_id" : 2, "scores" : [ 89, 90 ] }

The following operation adds new elements to the scores array, and then uses the $slice modifier to trim the array to the first three elements.以下操作将新元素添加到分数数组中，然后使用$slice修饰符将数组修剪为前三个元素。

db.students.updateOne(
   { _id: 2 },
   {
     $push: {
       scores: {
         $each: [ 100, 20 ],
         $slice: 3
       }
     }
   }
)

The result of the operation is to slice the elements of the updated scores array to the first three elements:操作的结果是将更新的scores数组的元素分片为前三个元素：

{ "_id" : 2, "scores" : [  89,  90,  100 ] }

Update Array Using Slice Only仅使用分片更新数组

A collection students contains the following document:students集合包含以下文档：

{ "_id" : 3, "scores" : [  89,  70,  100,  20 ] }

To update the scores field with just the effects of the $slice modifier, specify the number of elements to slice (e.g. -3) for the $slice modifier and an empty array [] for the $each modifier, as in the following:要仅使用$slice修饰符的效果更新scores字段，请为$slice修饰符指定要分片的元素数（例如-3），为$each修改器指定空数组[]，如下所示：

db.students.updateOne(
  { _id: 3 },
  {
    $push: {
      scores: {
         $each: [ ],
         $slice: -3
      }
    }
  }
)

The result of the operation is to slice the elements of the scores array to the last three elements:操作的结果是将scores数组的元素分片为最后三个元素：

{ "_id" : 3, "scores" : [  70,  100,  20 ] }

Use $slice with Other $push Modifiers将$slice与其他$push修饰符一起使用

Add the following document to the students collection:将以下文档添加到students集合中：

db.students.insertOne(
   {
      "_id" : 5,
      "quizzes" : [
         { "wk": 1, "score" : 10 },
         { "wk": 2, "score" : 8 },
         { "wk": 3, "score" : 5 },
         { "wk": 4, "score" : 6 }
      ]
   }
)

The following $push operation uses:以下$push操作使用：

  • the $each modifier to add multiple documents to the quizzes array,$each修饰符将多个文档添加到quizzes数组，
  • the $sort modifier to sort all the elements of the modified quizzes array by the score field in descending order, and$sort修饰符，用于按score字段降序对修改后的quizzes数组的所有元素进行排序，以及
  • the $slice modifier to keep only the first three sorted elements of the quizzes array.$slice修饰符只保留quizzes数组的前三个排序元素。
db.students.updateOne(
   { _id: 5 },
   {
     $push: {
       quizzes: {
          $each: [ { wk: 5, score: 8 }, { wk: 6, score: 7 }, { wk: 7, score: 6 } ],
          $sort: { score: -1 },
          $slice: 3
       }
     }
   }
)

After the operation only the three highest scoring quizzes are in the array:手术后，数组中只有三个得分最高的测验：

{
  "_id" : 5,
  "quizzes" : [
     { "wk" : 1, "score" : 10 },
     { "wk" : 2, "score" : 8 },
     { "wk" : 5, "score" : 8 }
  ]
}

The order of the modifiers is immaterial to the order in which the modifiers are processed. 修饰语的顺序与修饰语的处理顺序无关。See Modifiers for details.有关详细信息，请参阅修饰符。