MongoDB – $slice Modifier
Last Updated :
05 Jun, 2022
MongoDB provides different types of array update operators to update the values of the array fields in the documents and $slice modifier is one of them. This modifier is used to limit the number of array items during a $push operation. Syntax:
{
$push: {
<field>: {
$each: [ <value1>, <value2>, ... ],
$slice: <number>
}
}
}
- If the value of number is zero, then this modifier will update the array field to an empty array.
- If the value of number is negative, then this modifier will update the array field to contain only the last number items.
- If the value of number is positive, then this modifier will update the array field to contain only the first number items.
- $slice modifier must appear with $each modifier in the $push operator. You are allowed to pass an empty array in the $each modifier which help $slice modifier to show its effect. If you use the $slice modifier without $each modifier, then you will get an error.
- The order in which the modifiers appear in $push operation is immaterial.
In the following examples, we are working with:
Database: GeeksforGeeks Collection: contributor Document: two documents that contain the details of the contributor in the form of field-value pairs.
Slicing the array from the end:
In this example, we are adding new items in the language field and then use $slice modifier to trim the array to the last four items.
Python3
db.contributor.update({name: "Suman"},
{$push: {language: { $each: ["SQL", "JS + + "],
$ slice : - 4 }}})
|
Slicing the array from the front:
In this example, we are adding new items in the language field and then use $slice modifier to trim the array to the first five items.
Python3
db.contributor.update({name: "Rohit"},
{$push: {language: { $each: ["R Language", "JS + + "],
$ slice : 5 }}})
|
Updating an array using $slice modifier:
In this example, we are updating the array of the language field by trimming the array to the last three items.
Python3
db.contributor.update({name: "Rohit"},
{$push: {language: { $each: [],
$ slice : - 3 }}})
|
Using $slice modifier with other modifiers with $push operator:
In this example, we are using the $slice modifier with other modifiers like $each and $sort with $push operator.
Python3
db.contributor.update({name: "Rohit"},
{$push: { language: { $each: ["C + + ", "Kotlin"],
$sort: 1 , $ slice : 4 }}})
|
Here,
- The $each modifier is used to add multiple documents to the language array.
- The $sort modifier is used to sort all the items of the modified language array in ascending.
- The $slice modifier is used to keep only the first four sorted items of the language array.
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