- Reference >
- Operators >
- Update Operators >
- Array Update Operators >
- $slice
$slice¶
- $slice¶
2.4 新版功能.
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:
Value Description 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.
2.6 新版功能.
Behavior¶
在 2.6 版更改.
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.
Examples¶
Slice from the End of the Array¶
A collection students contains the following document:
{ "_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.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:
{ "_id" : 1, "scores" : [ 50, 60, 80, 78, 86 ] }
Slice from the Front of the Array¶
A collection students contains the following document:
{ "_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.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:
{ "_id" : 2, "scores" : [ 89, 90, 100 ] }
Update Array Using Slice Only¶
A collection students contains the following document:
{ "_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.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:
{ "_id" : 3, "scores" : [ 70, 100, 20 ] }
Use $slice with Other $push Modifiers¶
A collection students has the following document:
{
"_id" : 5,
"quizzes" : [
{ "wk": 1, "score" : 10 },
{ "wk": 2, "score" : 8 },
{ "wk": 3, "score" : 5 },
{ "wk": 4, "score" : 6 }
]
}
The following $push operation uses:
- the $each modifier to add multiple documents to the quizzes array,
- 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.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:
{
"_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.