Query for documents where array size is greater than 1 in MongoDB?

There are multiple ways to query documents where an array field has more than a certain number of elements in MongoDB. The recommended approaches are $expr with $size, or the dot notation trick with $exists.

Sample Data

db.students.insertMany([
    {"StudentName": "Larry", "Subjects": ["Java", "C", "C++"]},
    {"StudentName": "Maxwell", "Subjects": ["MongoDB"]},
    {"StudentName": "Carol", "Subjects": ["MySQL", "SQL Server", "PL/SQL"]}
]);

Method 1: Using $expr with $size (Recommended)

db.students.find({
    $expr: { $gt: [{ $size: "$Subjects" }, 1] }
}).pretty();

{ "StudentName": "Larry", "Subjects": ["Java", "C", "C++"] }
{ "StudentName": "Carol", "Subjects": ["MySQL", "SQL Server", "PL/SQL"] }

Maxwell's document (1 subject) is excluded because the array size is not greater than 1.

Method 2: Using Dot Notation with $exists

Check if the element at index 1 (second element) exists ?

db.students.find({
    "Subjects.1": { $exists: true }
}).pretty();

This returns documents where the array has at least 2 elements (index 1 exists).

Method 3: Using $where (Slower)

db.students.find({
    $where: "this.Subjects.length > 1"
}).pretty();

Note: $where executes JavaScript and is slower than native operators. Avoid for large collections.

Comparison

Conclusion

Use $expr with $size for flexible array length comparisons (MongoDB 3.6+). The dot notation trick ("array.N": {$exists: true}) is the fastest and works on all versions. Avoid $where for performance-critical queries.