How to update MongoDB collection using $toLower?

To update a MongoDB collection using $toLower functionality, use the forEach() method to iterate through documents and apply JavaScript's toLowerCase() function. While $toLower is primarily an aggregation operator, this approach allows you to permanently update field values in your collection.

Syntax

db.collection.find().forEach(
    function(document) {
        document.fieldName = document.fieldName.toLowerCase();
        db.collection.save(document);
    }
);

Sample Data

First, let's create a collection with mixed-case student names ?

db.toLowerDemo.insertMany([
    {"StudentId": 101, "StudentName": "John"},
    {"StudentId": 102, "StudentName": "Larry"},
    {"StudentId": 103, "StudentName": "CHris"},
    {"StudentId": 104, "StudentName": "ROBERT"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5c9b1b4515e86fd1496b38bf"),
        ObjectId("5c9b1b4b15e86fd1496b38c0"),
        ObjectId("5c9b1b5115e86fd1496b38c1"),
        ObjectId("5c9b1b5a15e86fd1496b38c2")
    ]
}

Display Original Data

db.toLowerDemo.find().pretty();

{
    "_id": ObjectId("5c9b1b4515e86fd1496b38bf"),
    "StudentId": 101,
    "StudentName": "John"
}
{
    "_id": ObjectId("5c9b1b4b15e86fd1496b38c0"),
    "StudentId": 102,
    "StudentName": "Larry"
}
{
    "_id": ObjectId("5c9b1b5115e86fd1496b38c1"),
    "StudentId": 103,
    "StudentName": "CHris"
}
{
    "_id": ObjectId("5c9b1b5a15e86fd1496b38c2"),
    "StudentId": 104,
    "StudentName": "ROBERT"
}

Update Using forEach() Method

Apply lowercase transformation to all StudentName fields ?

db.toLowerDemo.find().forEach(
    function(lower) {
        lower.StudentName = lower.StudentName.toLowerCase();
        db.toLowerDemo.save(lower);
    }
);

Verify Results

db.toLowerDemo.find().pretty();

{
    "_id": ObjectId("5c9b1b4515e86fd1496b38bf"),
    "StudentId": 101,
    "StudentName": "john"
}
{
    "_id": ObjectId("5c9b1b4b15e86fd1496b38c0"),
    "StudentId": 102,
    "StudentName": "larry"
}
{
    "_id": ObjectId("5c9b1b5115e86fd1496b38c1"),
    "StudentId": 103,
    "StudentName": "chris"
}
{
    "_id": ObjectId("5c9b1b5a15e86fd1496b38c2"),
    "StudentId": 104,
    "StudentName": "robert"
}

Key Points

• The forEach() method iterates through each document in the collection
• JavaScript's toLowerCase() function converts strings to lowercase
• The save() method updates the document with the modified field value
• This approach permanently modifies the actual collection data

Conclusion

While $toLower is an aggregation operator, you can achieve similar results for permanent updates using forEach() with JavaScript's toLowerCase(). This method iterates through documents and saves the modified values back to the collection.