MongoDB – $slice Modifier

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.