Set filtering conditions for nested array in MongoDB

To set filtering conditions for nested arrays in MongoDB, use the $filter operator combined with $anyElementTrue and $map in an aggregation pipeline. This approach allows you to filter outer array elements based on conditions within deeply nested arrays.

Syntax

db.collection.aggregate([
    {
        $addFields: {
            "arrayField": {
                $filter: {
                    input: "$arrayField",
                    as: "element",
                    cond: {
                        $anyElementTrue: {
                            $map: {
                                input: "$$element.nestedArray",
                                in: { condition_expression }
                            }
                        }
                    }
                }
            }
        }
    }
]);

Sample Data

db.demo725.insertOne({
    "details": {
        "userMessages": [
            {
                "Messages": [
                    { "Message": "Hello" },
                    { "Message": "How" },
                    { "Message": "are" }
                ]
            },
            {
                "Messages": [
                    { "Message": "Good" },
                    { "Message": "Bye" }
                ]
            },
            {
                "Messages": [
                    { "Message": "Hello" },
                    { "Message": "Bye" }
                ]
            }
        ]
    }
});

Example: Filter Messages Containing "Hello"

Filter userMessages array to keep only elements that contain "Hello" in any of their nested Messages ?

db.demo725.aggregate([
    {
        $addFields: {
            "details.userMessages": {
                $filter: {
                    input: "$details.userMessages",
                    as: "out",
                    cond: {
                        $anyElementTrue: {
                            $map: {
                                input: "$$out.Messages",
                                in: { $gte: [ { $indexOfBytes: [ "$$this.Message", "Hello" ] }, 0 ] }
                            }
                        }
                    }
                }
            }
        }
    }
]);

{
    "_id" : ObjectId("5eab16cd43417811278f5893"),
    "details" : {
        "userMessages" : [
            {
                "Messages" : [
                    { "Message" : "Hello" },
                    { "Message" : "How" },
                    { "Message" : "are" }
                ]
            },
            {
                "Messages" : [
                    { "Message" : "Hello" },
                    { "Message" : "Bye" }
                ]
            }
        ]
    }
}

How It Works

• $filter evaluates each element in the outer array against a condition
• $map transforms each nested array element to check the condition
• $anyElementTrue returns true if any nested element meets the criteria
• $indexOfBytes searches for "Hello" substring, returning -1 if not found

Conclusion

Combining $filter, $map, and $anyElementTrue enables complex filtering of nested arrays based on conditions within deeply nested elements. This pattern is essential for working with multi-level document structures in MongoDB.