What is the $unwind operator in MongoDB?

The $unwind operator in MongoDB deconstructs an array field from input documents to output a document for each element. Each output document contains the same fields as the original document, but the array field is replaced with a single array element.

Syntax

db.collection.aggregate([
    { $unwind: "$arrayField" }
]);

Sample Data

Let us create a collection with a document containing an array ?

db.unwindOperatorDemo.insertOne({
    "StudentName": "Larry",
    "StudentAge": 23,
    "StudentSubject": ["C", "C++", "Java", "MongoDB"]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5c7ef5f3559dd2396bcfbfc8")
}

Display the document to see the original structure ?

db.unwindOperatorDemo.find().pretty();

{
    "_id": ObjectId("5c7ef5f3559dd2396bcfbfc8"),
    "StudentName": "Larry",
    "StudentAge": 23,
    "StudentSubject": [
        "C",
        "C++",
        "Java",
        "MongoDB"
    ]
}

Example: Using $unwind Operator

Apply the $unwind operator to deconstruct the StudentSubject array ?

db.unwindOperatorDemo.aggregate([
    { $project: {
        StudentName: 1,
        StudentAge: 1,
        StudentSubject: 1
    }},
    { $unwind: "$StudentSubject" }
]).pretty();

{
    "_id": ObjectId("5c7ef5f3559dd2396bcfbfc8"),
    "StudentName": "Larry",
    "StudentAge": 23,
    "StudentSubject": "C"
}
{
    "_id": ObjectId("5c7ef5f3559dd2396bcfbfc8"),
    "StudentName": "Larry",
    "StudentAge": 23,
    "StudentSubject": "C++"
}
{
    "_id": ObjectId("5c7ef5f3559dd2396bcfbfc8"),
    "StudentName": "Larry",
    "StudentAge": 23,
    "StudentSubject": "Java"
}
{
    "_id": ObjectId("5c7ef5f3559dd2396bcfbfc8"),
    "StudentName": "Larry",
    "StudentAge": 23,
    "StudentSubject": "MongoDB"
}

How It Works

The $unwind operator transforms one document with an array into multiple documents. Each output document contains the original fields (StudentName and StudentAge) paired with a single element from the array field (StudentSubject).

Conclusion

The $unwind operator is essential for processing array fields in aggregation pipelines. It creates separate documents for each array element, enabling further operations on individual array values.