How do I return a document with filtered sub-documents using Mongo?

To return a document with filtered sub-documents in MongoDB, use the $project stage with $filter operator in an aggregation pipeline. The $filter operator allows you to select specific array elements based on conditions while preserving the document structure.

Syntax

db.collection.aggregate([
    {
        $project: {
            arrayField: {
                $filter: {
                    input: "$arrayField",
                    as: "item",
                    cond: { condition }
                }
            }
        }
    }
])

Create Sample Data

db.demo457.insertMany([
    {
        _id: 101,
        details: [
            { ProductName: "Product-1", ProductPrice: 90 },
            { ProductName: "Product-2", ProductPrice: 190 }
        ]
    },
    {
        _id: 102,
        details: [
            { ProductName: "Product-3", ProductPrice: 150 },
            { ProductName: "Product-4", ProductPrice: 360 }
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": {
        "0": 101,
        "1": 102
    }
}

View All Documents

db.demo457.find();

{ "_id": 101, "details": [ { "ProductName": "Product-1", "ProductPrice": 90 }, { "ProductName": "Product-2", "ProductPrice": 190 } ] }
{ "_id": 102, "details": [ { "ProductName": "Product-3", "ProductPrice": 150 }, { "ProductName": "Product-4", "ProductPrice": 360 } ] }

Example: Filter Products by Price ? 170

db.demo457.aggregate([
    {
        $project: {
            details: {
                $filter: {
                    input: "$details",
                    as: "output",
                    cond: { $gte: [ "$$output.ProductPrice", 170 ] }
                }
            }
        }
    }
]);

{ "_id": 101, "details": [ { "ProductName": "Product-2", "ProductPrice": 190 } ] }
{ "_id": 102, "details": [ { "ProductName": "Product-4", "ProductPrice": 360 } ] }

How It Works

• input − The array field to filter ("$details")
• as − Variable name for each array element ("output")
• cond − Filter condition using "$$output" to reference current element

Conclusion

Use $project with $filter to return documents containing only array elements that match specific criteria. This preserves the original document structure while filtering sub-document arrays based on conditions.