MongoDB aggregation framework to sort by length of array?

To sort documents by the length of an array field in MongoDB, use the aggregation framework with $addFields and $size operators to calculate array lengths, then sort by the computed field.

Syntax

db.collection.aggregate([
    { $addFields: { arrayLength: { $size: "$arrayField" } } },
    { $sort: { arrayLength: -1 } }
]);

Sample Data

db.demo33.insertMany([
    { "ListOfStudent": ["Chris", "Bob"] },
    { "ListOfStudent": ["David", "Adam", "Mike"] },
    { "ListOfStudent": ["Carol", "Sam", "John", "Robert"] }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e17556ccfb11e5c34d898ca"),
        ObjectId("5e17557acfb11e5c34d898cb"),
        ObjectId("5e1755a3cfb11e5c34d898cc")
    ]
}

Display Sample Data

db.demo33.find();

{ "_id": ObjectId("5e17556ccfb11e5c34d898ca"), "ListOfStudent": ["Chris", "Bob"] }
{ "_id": ObjectId("5e17557acfb11e5c34d898cb"), "ListOfStudent": ["David", "Adam", "Mike"] }
{ "_id": ObjectId("5e1755a3cfb11e5c34d898cc"), "ListOfStudent": ["Carol", "Sam", "John", "Robert"] }

Method 1: Using $size (Recommended)

Sort documents by array length in descending order using $size to calculate array length ?

db.demo33.aggregate([
    { $addFields: { studentCount: { $size: "$ListOfStudent" } } },
    { $sort: { studentCount: -1 } }
]);

{ "_id": ObjectId("5e1755a3cfb11e5c34d898cc"), "ListOfStudent": ["Carol", "Sam", "John", "Robert"], "studentCount": 4 }
{ "_id": ObjectId("5e17557acfb11e5c34d898cb"), "ListOfStudent": ["David", "Adam", "Mike"], "studentCount": 3 }
{ "_id": ObjectId("5e17556ccfb11e5c34d898ca"), "ListOfStudent": ["Chris", "Bob"], "studentCount": 2 }

Method 2: Using $unwind and $group

Alternative approach using $unwind to deconstruct array and $group to count elements ?

db.demo33.aggregate([
    { $unwind: "$ListOfStudent" },
    { $group: { _id: "$_id", ct: { $sum: 1 } } },
    { $sort: { ct: -1 } }
]);

{ "_id": ObjectId("5e1755a3cfb11e5c34d898cc"), "ct": 4 }
{ "_id": ObjectId("5e17557acfb11e5c34d898cb"), "ct": 3 }
{ "_id": ObjectId("5e17556ccfb11e5c34d898ca"), "ct": 2 }

Key Differences

• $size method: Preserves original document structure and adds array length field
• $unwind method: Returns only document ID and count, losing original array data
• $size is more efficient for simple length-based sorting

Conclusion

Use $size with $addFields to sort by array length while preserving document structure. The $unwind approach works but loses original data, making $size the preferred method for most use cases.