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Array Update Operators

$sort

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  • Behavior
  • Examples
$sort

The $sort modifier orders the elements of an array during a $push operation.

To use the $sort modifier, it must appear with the $each modifier. You can pass an empty array [] to the $each modifier such that only the $sort modifier has an effect.

{
  $push: {
     <field>: {
       $each: [ <value1>, <value2>, ... ],
       $sort: <sort specification>
     }
  }
}

For <sort specification>:

  • To sort array elements that are not documents, or if the array elements are documents, to sort by the whole documents, specify 1 for ascending or -1 for descending.

  • If the array elements are documents, to sort by a field in the documents, specify a sort document with the field and the direction, i.e. { field: 1 } or { field: -1 }. Do not reference the containing array field in the sort specification (e.g. { "arrayField.field": 1 } is incorrect).

Behavior

In MongoDB 4.4 and earlier, update operators process document fields in lexicographic order. See Update Operators Behavior for details.

The $sort modifier can sort array elements that are not documents. In previous versions, the $sort modifier required the array elements be documents.

If the array elements are documents, the modifier can sort by either the whole document or by a specific field in the documents. In previous versions, the $sort modifier can only sort by a specific field in the documents.

Trying to use the $sort modifier without the $each modifier results in an error. The $sort no longer requires the $slice modifier. For a list of modifiers available for $push, see Modifiers.

Examples

Sort Array of Documents by a Field in the Documents

A collection students contains the following document:

{
  "_id": 1,
  "quizzes": [
    { "id" : 1, "score" : 6 },
    { "id" : 2, "score" : 9 }
  ]
}

The following update appends additional documents to the quizzes array and then sorts all the elements of the array by the ascending score field:

db.students.update(
   { _id: 1 },
   {
     $push: {
       quizzes: {
         $each: [ { id: 3, score: 8 }, { id: 4, score: 7 }, { id: 5, score: 6 } ],
         $sort: { score: 1 }
       }
     }
   }
)

Important

The sort document refers directly to the field in the documents and does not reference the containing array field quizzes; i.e. { score: 1 } and not { "quizzes.score": 1}

After the update, the array elements are in order of ascending score field:

{
  "_id" : 1,
  "quizzes" : [
     { "id" : 1, "score" : 6 },
     { "id" : 5, "score" : 6 },
     { "id" : 4, "score" : 7 },
     { "id" : 3, "score" : 8 },
     { "id" : 2, "score" : 9 }
  ]
}

Sort Array Elements That Are Not Documents

A collection students contains the following document:

{ "_id" : 2, "tests" : [  89,  70,  89,  50 ] }

The following operation adds two more elements to the tests array and sorts the elements:

db.students.update(
   { _id: 2 },
   { $push: { tests: { $each: [ 40, 60 ], $sort: 1 } } }
)

The updated document has the elements of the tests array in ascending order:

{ "_id" : 2, "tests" : [  40,  50,  60,  70,  89,  89 ] }

Update Array Using Sort Only

A collection students contains the following document:

{ "_id" : 3, "tests" : [  89,  70,  100,  20 ] }

To update the tests field to sort its elements in descending order, specify the { $sort: -1 } and specify an empty array [] for the $each modifier. For example:

db.students.update(
   { _id: 3 },
   { $push: { tests: { $each: [ ], $sort: -1 } } }
)

The example sorts the tests field values in descending order:

{ "_id" : 3, "tests" : [ 100,  89,  70,  20 ] }

Use $sort 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 in the query does not change the order that the modifiers are applied. For details, see Modifiers.

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