MongoDB query to replace value with aggregation?

To replace values in MongoDB using aggregation, use the $project stage with the $literal operator. The $literal operator returns a specified value without interpreting it as an expression, making it perfect for replacing field values with static content.

Syntax

db.collection.aggregate([
    {
        $project: {
            fieldName: { $literal: "newValue" },
            otherField: 1
        }
    }
]);

Sample Data

db.replaceValueDemo.insertMany([
    {
        _id: 100,
        "EmployeeName": "Chris",
        "EmployeeOtherDetails": {
            "EmployeeDesignation": "HR",
            "EmployeeAge": 27
        }
    },
    {
        _id: 101,
        "EmployeeName": "David",
        "EmployeeOtherDetails": {
            "EmployeeDesignation": "Tester",
            "EmployeeAge": 26
        }
    }
]);

{ "acknowledged": true, "insertedIds": { "0": 100, "1": 101 } }

Example: Replace Designation with Fixed Value

Replace all employee designations with "Developer" while keeping other fields ?

db.replaceValueDemo.aggregate([
    {
        $project: {
            "_id": 1,
            "EmployeeName": 1,
            "EmployeeOtherDetails": {
                "EmployeeAge": "$EmployeeOtherDetails.EmployeeAge",
                "EmployeeDesignation": { $literal: "Developer" }
            }
        }
    }
]);

{ "_id": 100, "EmployeeName": "Chris", "EmployeeOtherDetails": { "EmployeeAge": 27, "EmployeeDesignation": "Developer" } }
{ "_id": 101, "EmployeeName": "David", "EmployeeOtherDetails": { "EmployeeAge": 26, "EmployeeDesignation": "Developer" } }

Key Points

• $literal treats the value as a constant, not an expression or field reference.
• Use 1 to include existing fields unchanged.
• Nested field replacement requires reconstructing the parent object structure.

Conclusion

The $literal operator in aggregation pipelines effectively replaces field values with static content. This approach is ideal when you need to transform data without modifying the original collection.