MongoDB query to get only specific fields in nested array documents?

To get only specific fields in nested array documents in MongoDB, use $filter with $project in an aggregation pipeline to match conditions and extract required data from deeply nested arrays.

Syntax

db.collection.aggregate([
    {
        $project: {
            "field": {
                $filter: {
                    input: "$arrayField",
                    cond: { $eq: ["$$this.fieldName", "value"] }
                }
            }
        }
    }
]);

Sample Data

db.demo342.insertOne({
    "Id": "101",
    "details1": {
        "details2": [
            {
                "details3": [
                    {
                        "Name": "Mike",
                        "CountryName": "US"
                    },
                    {
                        "Name": "David",
                        "CountryName": "AUS"
                    },
                    {
                        "Name": "Bob",
                        "CountryName": "UK"
                    }
                ]
            }
        ]
    }
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e53ef99f8647eb59e5620a9")
}

Verify Sample Data

db.demo342.find();

{
    "_id": ObjectId("5e53ef99f8647eb59e5620a9"),
    "Id": "101",
    "details1": {
        "details2": [
            {
                "details3": [
                    { "Name": "Mike", "CountryName": "US" },
                    { "Name": "David", "CountryName": "AUS" },
                    { "Name": "Bob", "CountryName": "UK" }
                ]
            }
        ]
    }
}

Example: Filter Nested Array by Name

Get only documents where Name equals "Bob" from the deeply nested array ?

db.demo342.aggregate([
    {
        "$project": {
            "details1": {
                "details2": {
                    "$filter": {
                        "input": {
                            "$map": {
                                "input": "$details1.details2",
                                "in": {
                                    "details3": {
                                        "$filter": {
                                            "input": "$$this.details3",
                                            "cond": { "$eq": ["$$this.Name", "Bob"] }
                                        }
                                    }
                                }
                            }
                        },
                        "cond": { "$ne": ["$$this.details3", []] }
                    }
                }
            }
        }
    }
]);

{
    "_id": ObjectId("5e53ef99f8647eb59e5620a9"),
    "details1": {
        "details2": [
            {
                "details3": [
                    { "Name": "Bob", "CountryName": "UK" }
                ]
            }
        ]
    }
}

How It Works

• $map processes each element in the details2 array
• Inner $filter matches documents where Name equals "Bob"
• Outer $filter removes empty arrays using $ne condition
• $$this refers to the current element being processed

Conclusion

Use nested $filter operators with $map to extract specific documents from deeply nested arrays. This approach preserves the original document structure while showing only matching elements.