MongoDB - модификатор $ slice

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.

В следующих примерах мы работаем с:

Database: GeeksforGeeks
Collection: contributor
Document: two documents that contain the details of the contributor in the form of field-value pairs.

Нарезка массива с конца:

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.

Нарезка массива спереди:

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.

Updating an array using $slice modifier:

In this example, we are updating the array of the language field by triming the array to the last three items.

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.

Здесь,

• Модификатор $ each используется для добавления нескольких документов в языковой массив.
• Модификатор $ sort используется для сортировки всех элементов измененного языкового массива по возрастанию.
• Модификатор $ slice используется для сохранения только первых четырех отсортированных элементов языкового массива.