Include all existing fields and add new fields to document in MongoDB?

To include all existing fields and add new fields to a document in MongoDB, use the $addFields operator in an aggregation pipeline. This operator preserves all existing fields while adding new computed fields to the output.

Syntax

db.collection.aggregate([
    {
        $addFields: {
            "newField1": "value1",
            "newField2": "$existingField",
            "newField3": { $multiply: ["$field1", "$field2"] }
        }
    }
]);

Sample Data

db.addFieldDemo.insertOne({
    "EmployeeId": 101,
    "EmployeeName": "Larry",
    "EmployeeDetails": {
        "EmployeeSalary": 65000,
        "EmployeeCity": "New York",
        "Message": "Hi"
    }
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5c7f654d8d10a061296a3c44")
}

Display Current Document

db.addFieldDemo.find().pretty();

{
    "_id": ObjectId("5c7f654d8d10a061296a3c44"),
    "EmployeeId": 101,
    "EmployeeName": "Larry",
    "EmployeeDetails": {
        "EmployeeSalary": 65000,
        "EmployeeCity": "New York",
        "Message": "Hi"
    }
}

Example: Add New Field from Nested Document

Extract the salary from the nested document and add it as a new top-level field ?

db.addFieldDemo.aggregate([
    {
        $addFields: {
            "EmployeeBasicSalary": "$EmployeeDetails.EmployeeSalary"
        }
    }
]).pretty();

{
    "_id": ObjectId("5c7f654d8d10a061296a3c44"),
    "EmployeeId": 101,
    "EmployeeName": "Larry",
    "EmployeeDetails": {
        "EmployeeSalary": 65000,
        "EmployeeCity": "New York",
        "Message": "Hi"
    },
    "EmployeeBasicSalary": 65000
}

Key Points

• $addFields preserves all existing document fields unchanged.
• Use dot notation to reference nested field values like "$EmployeeDetails.EmployeeSalary".
• New fields can contain static values, field references, or computed expressions.

Conclusion

The $addFields operator in aggregation pipelines allows you to add new computed fields while keeping all existing document structure intact. This is ideal for data transformation without losing original information.