How to compare two fields in aggregation filter with MongoDB?

To compare two fields in an aggregation filter with MongoDB, use the $filter operator within a $project stage. This allows you to filter array elements based on field comparisons using the $eq operator.

Syntax

db.collection.aggregate([
    {
        $project: {
            arrayField: {
                $filter: {
                    input: "$arrayField",
                    as: "variable",
                    cond: { $eq: ["$$variable.field", "$otherField"] }
                }
            }
        }
    }
]);

Sample Data

db.demo137.insertOne({
    Name1: "Chris",
    Name2: "David",
    Detail1: [
        {_id: "John", Name3: "Chris"},
        {_id: "Chris", Name3: "Chris"}
    ],
    Detail2: [
        {_id: "David", Name3: "Chris"},
        {_id: "Carol", Name3: "Chris"},
        {_id: "John", Name3: "Chris"}
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e31b4cefdf09dd6d08539a0")
}

Display all documents from the collection ?

db.demo137.find();

{
    "_id": ObjectId("5e31b4cefdf09dd6d08539a0"),
    "Name1": "Chris",
    "Name2": "David",
    "Detail1": [
        { "_id": "John", "Name3": "Chris" },
        { "_id": "Chris", "Name3": "Chris" }
    ],
    "Detail2": [
        { "_id": "David", "Name3": "Chris" },
        { "_id": "Carol", "Name3": "Chris" },
        { "_id": "John", "Name3": "Chris" }
    ]
}

Example: Filter Arrays Based on Field Comparison

Filter Detail1 and Detail2 arrays to show only elements where the _id matches Name1 and Name2 respectively ?

db.demo137.aggregate([
    {
        $project: {
            _id: 0,
            Name1: 1,
            Name2: 1,
            Detail1: {
                $filter: {
                    input: "$Detail1",
                    as: "out",
                    cond: {
                        $eq: ["$$out._id", "$Name1"]
                    }
                }
            },
            Detail2: {
                $filter: {
                    input: "$Detail2",
                    as: "out",
                    cond: {
                        $eq: ["$$out._id", "$Name2"]
                    }
                }
            }
        }
    }
]);

{
    "Name1": "Chris",
    "Name2": "David",
    "Detail1": [
        { "_id": "Chris", "Name3": "Chris" }
    ],
    "Detail2": [
        { "_id": "David", "Name3": "Chris" }
    ]
}

Key Points

• Use $$variable to reference the current element in the $filter condition.
• Use $fieldName to reference document-level fields for comparison.
• The $eq operator performs exact field-to-field comparison within the filter condition.

Conclusion

The $filter operator with $eq effectively compares fields within aggregation pipelines. This approach filters array elements by matching their properties against other document fields, providing precise control over query results.