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$sort

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$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).

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 $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.

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 }
]
}

A collection students contains the following document:

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

The following operation adds two more elements to the scores 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 scores array in ascending order:

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

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, as in the following:

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

The result of the operation is to update the scores field to sort its elements in descending order:

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

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 is immaterial to the order in which the modifiers are processed. See Modifiers for details.

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