$slice

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$slice

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

To use the $slice modifier, it must appear with the $each modifier. 要使用$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 $slice modifier has an effect.可以将空数组[]传递给$each修饰符，以便只有$slice修饰符具有效果。

{
  $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消极的 To update the array <field> to contain only the last <num> elements.更新数组<field>以仅包含最后的<num>元素。
Positive正数 To update the array <field> contain only the first <num> elements.更新数组<field>以仅包含前<num>元素。

Behavior行为

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

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

Examples示例

Slice from the End of the Array从数组末尾切片

A collection students contains the following document:集合students包括以下文档：

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

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

db.students.update(
   { _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数组的元素切片为最后五个元素：

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

Slice from the Front of the Array从数组的前面切片

A collection students contains the following document:集合students包括以下文件：

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

The following operation adds new elements to the scores array, and then uses the $slice modifier to trim the array to the first three elements.下面的操作向scores数组添加新元素，然后使用$slice修饰符将数组修剪为前三个元素。

db.students.update(
   { _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后的scores数组的元素切片为前三个元素：

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

Update Array Using Slice Only仅使用切片更新数组

A collection students contains the following document:学生students包括以下文档：

{ "_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$slice修饰符指定要切片的元素数（例如-3），为$each修饰符指定空数组[]，如下所示：

db.students.update(
  { _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数组的元素切片为最后三个元素：

{ "_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

A collection students has the following document:集合students有以下文档：

{
   "_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数组中，
  • 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字段降序排列修改后的quizzes数组的所有元素，以及
  • the $slice modifier to keep only the first three sorted elements of the quizzes array.$slice修饰符只保留测验数组的前三个排序元素。
db.students.update(
   { _id: 5 },
   {
     $push: {
       quizzes: {
          $each: [ { wk: 5, score: 8 }, { wk: 6, score: 7 }, { wk: 7, score: 6 } ],
          $sort: { score: -1 },
          $slice: 3
       }
     }
   }
)

The result of the operation is keep only the three highest scoring quizzes:该操作的结果是只保留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。