MongoDB $unwind Operator – (Aggregation Stage)

In this tutorial, we will learn about an aggregation stage MongoDB $unwind operator. To understand this topic we assume that you have a basic understanding of how to fetch or read documents from a collection.

Before going further, please make sure to install MongoDB on your computer.

What is MongoDB $unwind Operator

Definition

MongoDB $unwind operator is used to deconstructing an array field from the input documents to output a document for each element. Each output document is the input document with the value of the array field replaced by the element.

Simply you can say the MongoDB $unwind duplicates each document in the pipeline, once per array element.

To understand the concept of $unwind operator, let’s take an example and understand it step by step.

Let’s create a users collection with the document. To create a collection with a document the query is like:

db.users.insertOne(
    {
        "name": "Avinash",
        "age": 27,
        "company": "Techy Hunger",
        "skills": ["Java", "MongoDB", "Node", "Angular"]
    }
);

Now display documents from users collection using find() method.

db.users.find({}).pretty()

{
    "_id" : ObjectId("5d958cdc446a46c989b98887"),
    "name" : "Avinash",
    "age" : 27.0,
    "company" : "Techy Hunger",
    "skills" : [ 
        "Java", 
        "MongoDB", 
        "Node", 
        "Angular"
    ]
}

db.users.aggregate({
    $project: {
        name: 1,
        age: 1,
        skills: 1
    }},
    { $unwind: "$skills" }
)

/* 1 */
{
    "_id" : ObjectId("5d958cdc446a46c989b98887"),
    "name" : "Avinash",
    "age" : 27.0,
    "skills" : "Java"
}

/* 2 */
{
    "_id" : ObjectId("5d958cdc446a46c989b98887"),
    "name" : "Avinash",
    "age" : 27.0,
    "skills" : "MongoDB"
}

/* 3 */
{
    "_id" : ObjectId("5d958cdc446a46c989b98887"),
    "name" : "Avinash",
    "age" : 27.0,
    "skills" : "Node"
}

/* 4 */
{
    "_id" : ObjectId("5d958cdc446a46c989b98887"),
    "name" : "Avinash",
    "age" : 27.0,
    "skills" : "Angular"
}

MongoDB $unwind Nested Array

Let’s insert one more document into products collection using the below code:

db.products.insertOne({
    "name": "Test",
    "items": [{
        "quantity": "10",
        "category": "test_cat",
        "images": [{
                "name": "abc.PNG",
                "size": 98
            },
            {
                "name": "def.PNG",
                "size": 156
            }
        ],
        "link": "http://url_of_link"
    }]
})

We will use $unwind operator here on images, See the code below:

db.products.aggregate({

        $project: {
            name: 1,
            items: 1
        }
    },
    [{
        '$unwind': {
            'path': '$items'
        }
    }, {
        '$unwind': {
            'path': '$items.images'
        }
    }]
)

Conclusion

In this tutorial, you have learned about MongoDB $unwind operator. I will suggest you to keep practicing each and every step carefully and you will be master in it.

I hope you liked this tutorial. Feel free to ask any doubt in the comment section below, I will be happy to resolve your queries.