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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.

To use the $slice modifier, it must appear with the $each modifier. You can pass an empty array [] to the $each modifier such that only the $slice modifier has an effect.

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

The <num> can be:

설명

Zero

To update the array <field> to an empty array.

Negative

To update the array <field> to contain only the last <num> elements.

Positive

To update the array <field> contain only the first <num> elements.

MongoDB 5.0부터 업데이트 연산자는 문자열 기반 이름이 있는 문서 필드를 사전순으로 처리합니다. 숫자 이름이 있는 필드는 숫자 순서대로 처리됩니다. 자세한 내용은 업데이트 운영자 동작을 참조하십시오.

The order in which the modifiers appear is immaterial. 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.

Trying to use the $slice modifier without the $each modifier results in an error.

컬렉션 students에는 다음 문서가 포함되어 있습니다.

{ "_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:

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:

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

컬렉션 students에는 다음 문서가 포함되어 있습니다.

{ "_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.

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:

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

컬렉션 students에는 다음 문서가 포함되어 있습니다.

{ "_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:

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:

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

다음 문서를 students 컬렉션에 추가합니다:

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

다음 $push 작업에서는 다음을 사용합니다.

  • quizzes 배열에 여러 문서를 추가하는 $each 수정자,

  • the $sort modifier to sort all the elements of the modified quizzes array by the score field in descending order, and

  • the $slice modifier to keep only the first three sorted elements of the quizzes array.

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
}
}
}
)

작업 후에는 가장 높은 점수를 받은 세 개의 퀴즈만 배열에 표시됩니다:

{
"_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. See Modifiers for details.

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