Update MongoDB field using value of another field?

In MongoDB, you can update a field using the value of another field in the same document. There are two main approaches − using the aggregation pipeline with $set (MongoDB 4.2+) or using $addFields with $out for writing results to a collection.

Method 1: Update with Aggregation Pipeline (MongoDB 4.2+)

Use updateMany() with an aggregation pipeline to set a field based on other fields ?

// Create sample data
db.studentInformation.insertOne({
    "StudentFirstName": "Carol",
    "StudentLastName": "Taylor"
});

// Update: create FullName from FirstName + LastName
db.studentInformation.updateMany(
    {},
    [{ $set: { "FullName": { $concat: ["$StudentFirstName", " ", "$StudentLastName"] } } }]
);

// Verify
db.studentInformation.find().pretty();

{
    "_id": ObjectId("..."),
    "StudentFirstName": "Carol",
    "StudentLastName": "Taylor",
    "FullName": "Carol Taylor"
}

Method 2: Using $addFields with $out

$addFields creates a computed field, and $out writes the result to a collection ?

db.studentInformation.aggregate([
    {
        $addFields: {
            "FullName": { $concat: ["$StudentFirstName", " ", "$StudentLastName"] }
        }
    },
    { $out: "studentInformation" }
]);

// Verify
db.studentInformation.find().pretty();

{
    "_id": ObjectId("..."),
    "StudentFirstName": "Carol",
    "StudentLastName": "Taylor",
    "FullName": "Carol Taylor"
}

Note: $out replaces the entire target collection. Use Method 1 (updateMany) for in-place updates without replacing the collection.

More Examples

Other common operations using one field's value to update another ?

// Copy one field to another
db.products.updateMany({}, [{ $set: { "backup_price": "$price" } }]);

// Math operations between fields
db.orders.updateMany({}, [{ $set: { "total": { $multiply: ["$quantity", "$unit_price"] } } }]);

// Conditional update
db.users.updateMany({}, [{
    $set: {
        "status": { $cond: { if: { $gte: ["$age", 18] }, then: "adult", else: "minor" } }
    }
}]);

Conclusion

Use updateMany() with an aggregation pipeline (MongoDB 4.2+) to update fields using values from other fields in the same document. Common expressions include $concat, $multiply, and $cond for string, math, and conditional operations respectively.