How to create double nested array in MongoDB?

To create a double nested array in MongoDB, use insertOne() or insertMany() with documents containing arrays of objects that themselves contain arrays. A double nested array means an array within an array structure.

Syntax

db.collection.insertOne({
    "field1": "value1",
    "outerArray": [
        {
            "innerField": "value",
            "innerArray": [
                { "nestedField": "value1" },
                { "nestedField": "value2" }
            ]
        }
    ]
});

Example 1: Student Project Database

Create a collection storing student details with nested project information ?

db.doubleNestedArrayDemo.insertOne({
    "StudentId": "1000",
    "StudentName": "Larry",
    "StudentDetails": [
        {
            "ProjectName": "Online Banking",
            "ProjectDetails": [
                {
                    "TechnologyUsed": "Java"
                },
                {
                    "TechnologyUsed": "MySQL in Backend"
                }
            ]
        }
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5c992bd7330fd0aa0d2fe4cc")
}

Example 2: Multiple Documents at Once

db.doubleNestedArrayDemo.insertMany([
    {
        "StudentId": "1001",
        "StudentName": "Robert",
        "StudentDetails": [
            {
                "ProjectName": "Student Web Tracker",
                "ProjectDetails": [
                    { "TechnologyUsed": "Django Framework" },
                    { "TechnologyUsed": "MongoDB in Backend" }
                ]
            }
        ]
    },
    {
        "StudentId": "1002",
        "StudentName": "Alice",
        "StudentDetails": [
            {
                "ProjectName": "E-commerce Platform",
                "ProjectDetails": [
                    { "TechnologyUsed": "React.js" },
                    { "TechnologyUsed": "Node.js" },
                    { "TechnologyUsed": "PostgreSQL" }
                ]
            }
        ]
    }
]);

Verify Results

Display all documents to verify the double nested array structure ?

db.doubleNestedArrayDemo.find().pretty();

{
    "_id": ObjectId("5c992bd7330fd0aa0d2fe4cc"),
    "StudentId": "1000",
    "StudentName": "Larry",
    "StudentDetails": [
        {
            "ProjectName": "Online Banking",
            "ProjectDetails": [
                {
                    "TechnologyUsed": "Java"
                },
                {
                    "TechnologyUsed": "MySQL in Backend"
                }
            ]
        }
    ]
}
{
    "_id": ObjectId("5c992cdb330fd0aa0d2fe4cd"),
    "StudentId": "1001",
    "StudentName": "Robert",
    "StudentDetails": [
        {
            "ProjectName": "Student Web Tracker",
            "ProjectDetails": [
                {
                    "TechnologyUsed": "Django Framework"
                },
                {
                    "TechnologyUsed": "MongoDB in Backend"
                }
            ]
        }
    ]
}

Conclusion

Double nested arrays in MongoDB are created by embedding arrays within array elements. This structure is ideal for storing hierarchical data like student projects with multiple technologies, product categories with subcategories, or any parent-child-grandchild relationships.