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Array Update Operators

$slice

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  • Behavior
  • Examples
$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:

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.

Behavior

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. Fields with numeric names are processed in numeric order. See Update Operators Behavior for details.

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.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 ] }

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.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 ] }

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.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 ] }

Use $slice with Other $push Modifiers

Add the following document to the students collection:

db.students.insertOne(
   {
      "_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.updateOne(
   { _id: 5 },
   {
     $push: {
       quizzes: {
          $each: [ { wk: 5, score: 8 }, { wk: 6, score: 7 }, { wk: 7, score: 6 } ],
          $sort: { score: -1 },
          $slice: 3
       }
     }
   }
)

After the operation only the three highest scoring quizzes are in the array:

{
  "_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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