Count the number of items in an array in MongoDB?

To count the number of items in an array in MongoDB, you can use the $size operator within the $project stage of an aggregation pipeline. The $size operator returns the number of elements in a specified array field. The syntax is −

db.yourCollectionName.aggregate({
    $project: {
        anyFieldName: { $size: "$yourArrayName" }
    }
}).pretty();

Creating a Sample Collection

To understand the above syntax, let us create a collection with documents. Each document contains a StudentMarks array with a different number of elements ?

db.getSizeOfArray.insertOne({
    "StudentId": 1,
    "StudentName": "Larry",
    "StudentMarks": [87, 34, 56, 77, 89, 90]
});

db.getSizeOfArray.insertOne({
    "StudentId": 2,
    "StudentName": "Sam",
    "StudentMarks": [90, 76, 56]
});

db.getSizeOfArray.insertOne({
    "StudentId": 3,
    "StudentName": "Carol",
    "StudentMarks": [90, 76]
});

Now display all documents from the collection using the find() method −

db.getSizeOfArray.find().pretty();

The following is the output −

{
    "_id" : ObjectId("5c6ebc536fd07954a4890680"),
    "StudentId" : 1,
    "StudentName" : "Larry",
    "StudentMarks" : [ 87, 34, 56, 77, 89, 90 ]
}
{
    "_id" : ObjectId("5c6ebc6b6fd07954a4890681"),
    "StudentId" : 2,
    "StudentName" : "Sam",
    "StudentMarks" : [ 90, 76, 56 ]
}
{
    "_id" : ObjectId("5c6ebc7a6fd07954a4890682"),
    "StudentId" : 3,
    "StudentName" : "Carol",
    "StudentMarks" : [ 90, 76 ]
}

Counting Array Items Using $size

The following query uses the $size operator to count the number of items in the StudentMarks array for each document −

db.getSizeOfArray.aggregate({
    $project: {
        NumberOfItemsInArray: { $size: "$StudentMarks" }
    }
}).pretty();

The following is the output −

{
    "_id" : ObjectId("5c6ebc536fd07954a4890680"),
    "NumberOfItemsInArray" : 6
}
{
    "_id" : ObjectId("5c6ebc6b6fd07954a4890681"),
    "NumberOfItemsInArray" : 3
}
{
    "_id" : ObjectId("5c6ebc7a6fd07954a4890682"),
    "NumberOfItemsInArray" : 2
}

Larry has 6 marks in the array, Sam has 3, and Carol has 2. The $size operator correctly returns the count for each document's array.

Conclusion

Use the $size operator within a $project aggregation stage to count the number of items in an array field in MongoDB. The operator takes the array field name prefixed with $ and returns the element count for each document.