How to Sort Array in MongoDB: Syntax and Examples

To sort an array inside a MongoDB document, use the $sortArray operator within an aggregation pipeline stage like $project or $set. For updating arrays, use the $push operator with $each and $sort modifiers to reorder array elements.

Syntax

MongoDB provides different ways to sort arrays inside documents depending on the operation:

• Aggregation Pipeline: Use $sortArray (MongoDB 5.2+) or $sort inside $project or $set to reorder array elements.
• Update Operation: Use $push with $each and $sort modifiers to sort an array during update.

Example syntax for aggregation:

{ $set: { sortedArray: { $sortArray: { input: "$arrayField", sortBy: 1 } } } }

Example syntax for update:

{ $push: { arrayField: { $each: [], $sort: 1 } } }

mongodb

{
  $set: {
    sortedArray: { $sortArray: { input: "$arrayField", sortBy: 1 } }
  }
}

Example

This example shows how to sort an array field named scores inside documents using an aggregation pipeline with $set and $sortArray. It sorts the array in ascending order.

mongodb

db.students.aggregate([
  {
    $set: {
      sortedScores: { $sortArray: { input: "$scores", sortBy: 1 } }
    }
  }
])

Output

[
  {
    _id: ObjectId("..."),
    name: "Alice",
    scores: [88, 75, 92],
    sortedScores: [75, 88, 92]
  },
  {
    _id: ObjectId("..."),
    name: "Bob",
    scores: [95, 85, 90],
    sortedScores: [85, 90, 95]
  }
]

Common Pitfalls

Common mistakes when sorting arrays in MongoDB include:

• Trying to use $sort directly on arrays outside aggregation or update contexts.
• Not using $each with $push when sorting arrays during updates.
• Using older MongoDB versions that do not support $sortArray (added in 5.2).

Always check your MongoDB version and use the correct operator for your use case.

mongodb

/* Wrong: Trying to sort array directly in update without $each */
db.collection.updateOne(
  { _id: 1 },
  { $push: { scores: { $sort: 1 } } }
)

/* Right: Use $each with $sort */
db.collection.updateOne(
  { _id: 1 },
  { $push: { scores: { $each: [], $sort: 1 } } }
)

Quick Reference

Operation	Syntax Example	Description
Aggregation sort array	{ $set: { sorted: { $sortArray: { input: "$arr", sortBy: 1 } } } }	Sorts array ascending inside aggregation
Update sort array	{ $push: { arr: { $each: [], $sort: -1 } } }	Sorts array descending during update
Legacy aggregation	{ $project: { sorted: { $map: { input: { $sortArray: "$arr" }, as: "x", in: "$x" } } } }	Alternative sorting in older versions

Key Takeaways

Use $sortArray in aggregation pipelines to sort arrays inside documents.

Use $push with $each and $sort modifiers to sort arrays during updates.

Check MongoDB version; $sortArray requires version 5.2 or newer.

Avoid using $sort directly on arrays outside aggregation or update contexts.

Sorting arrays in MongoDB requires specific operators depending on the operation.