Querying internal array size in MongoDB?

To query internal array size in MongoDB, use the $size operator. This operator returns the number of elements in an array field and can be used within aggregation pipelines to count array elements for specific documents.

Syntax

db.collection.aggregate([
    {
        $group: {
            _id: yourObjectIdValue,
            fieldName: { $first: { $size: "$arrayFieldName" } }
        }
    }
]);

Sample Data

Let's create a collection with employee documents containing technology arrays ?

db.internalArraySizeDemo.insertMany([
    {
        "EmployeeName": "Mike",
        "EmployeeTechnology": ["Java Web Development", "Python Web Development"]
    },
    {
        "EmployeeName": "Sam", 
        "EmployeeTechnology": ["C with Graphics", "Game Development with C++ Language", "MatLab"]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5c6eff586fd07954a48906b2"),
        ObjectId("5c6effcd6fd07954a48906b3")
    ]
}

View Sample Data

db.internalArraySizeDemo.find().pretty();

{
    "_id": ObjectId("5c6eff586fd07954a48906b2"),
    "EmployeeName": "Mike",
    "EmployeeTechnology": [
        "Java Web Development",
        "Python Web Development"
    ]
}
{
    "_id": ObjectId("5c6effcd6fd07954a48906b3"),
    "EmployeeName": "Sam",
    "EmployeeTechnology": [
        "C with Graphics",
        "Game Development with C++ Language",
        "MatLab"
    ]
}

Example: Get Array Size for Specific Document

Query the array size for Mike's EmployeeTechnology array ?

db.internalArraySizeDemo.aggregate([
    {
        $group: {
            _id: ObjectId("5c6eff586fd07954a48906b2"),
            EmployeeTechnology_count: { $first: { $size: "$EmployeeTechnology" } }
        }
    }
]);

{ "_id": ObjectId("5c6eff586fd07954a48906b2"), "EmployeeTechnology_count": 2 }

Alternative: Using $project with $size

A more direct approach using $project to get array sizes for all documents ?

db.internalArraySizeDemo.aggregate([
    {
        $project: {
            EmployeeName: 1,
            TechnologyCount: { $size: "$EmployeeTechnology" }
        }
    }
]);

{ "_id": ObjectId("5c6eff586fd07954a48906b2"), "EmployeeName": "Mike", "TechnologyCount": 2 }
{ "_id": ObjectId("5c6effcd6fd07954a48906b3"), "EmployeeName": "Sam", "TechnologyCount": 3 }

Conclusion

The $size operator effectively counts array elements in MongoDB. Use it with $project for all documents or $group for specific document analysis within aggregation pipelines.