MongoDB $addToSet to add a deep nested array of object?

The $addToSet operator adds a value to an array unless the value is already present, in which case $addToSet does nothing to that array. When working with deep nested arrays, combine $addToSet with the positional operator $ to target specific array elements.

Syntax

db.collection.update(
    { "parentArray.field": "matchValue" },
    { $addToSet: { "parentArray.$.nestedArray": newObject } }
);

Sample Data

Let us first create a collection with documents ?

db.demo380.insertOne({
    "details": [
        {
            "Name": "Chris",
            "details1": []
        },
        {
            "Name": "David", 
            "details1": []
        }
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e5b56e32ae06a1609a00b11")
}

Display all documents from a collection with the help of find() method ?

db.demo380.find().pretty();

{
    "_id": ObjectId("5e5b56e32ae06a1609a00b11"),
    "details": [
        {
            "Name": "Chris",
            "details1": []
        },
        {
            "Name": "David",
            "details1": []
        }
    ]
}

Example: Add Object to Nested Array

Following is the query to add a deep nested array of object ?

db.demo380.update({
    "details.Name": "David"
}, {
    $addToSet: {
        "details.$.details1": {
            "SubjectName": "MongoDB",
            "TeacherName": "Bob"
        }
    }
});

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

Display all documents from a collection with the help of find() method ?

db.demo380.find().pretty();

{
    "_id": ObjectId("5e5b56e32ae06a1609a00b11"),
    "details": [
        {
            "Name": "Chris",
            "details1": []
        },
        {
            "Name": "David",
            "details1": [
                {
                    "SubjectName": "MongoDB",
                    "TeacherName": "Bob"
                }
            ]
        }
    ]
}

Key Points

• Use $ positional operator to identify the matched array element
• $addToSet prevents duplicate objects from being added to the nested array
• The query matches the parent array element by a specific field value

Conclusion

The $addToSet operator combined with the positional operator $ effectively adds objects to deep nested arrays while preventing duplicates. This approach ensures data integrity in complex nested document structures.