MongoDB query to change simple field into an object?

To change a simple field into an object in MongoDB, use the $rename operator with a temporary field approach. This involves renaming the field to a temporary name, then renaming it back with the desired object structure using dot notation.

Syntax

db.collection.updateMany(
    {},
    { $rename: { "fieldName": "tempField" } }
);

db.collection.updateMany(
    {},
    { $rename: { "tempField": "objectName.nestedField" } }
);

Sample Data

db.changeSimpleFieldDemo.insertOne({
    "StudentMarks": 58,
    "StudentSubject": "MySQL"
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e0847a825ddae1f53b62205")
}

Let us view the inserted document ?

db.changeSimpleFieldDemo.find();

{ "_id": ObjectId("5e0847a825ddae1f53b62205"), "StudentMarks": 58, "StudentSubject": "MySQL" }

Example: Transform Fields into Nested Object

Transform "StudentMarks" and "StudentSubject" into a nested "Student" object ?

db.changeSimpleFieldDemo.updateMany(
    {},
    { $rename: { "StudentMarks": "tempMarks", "StudentSubject": "tempSubject" } }
);

{ "acknowledged": true, "matchedCount": 1, "modifiedCount": 1 }

db.changeSimpleFieldDemo.updateMany(
    {},
    { $rename: { "tempMarks": "Student.Marks", "tempSubject": "Student.Subject" } }
);

{ "acknowledged": true, "matchedCount": 1, "modifiedCount": 1 }

Verify Result

db.changeSimpleFieldDemo.find();

{
    "_id": ObjectId("5e0847a825ddae1f53b62205"),
    "Student": {
        "Marks": 58,
        "Subject": "MySQL"
    }
}

Key Points

• Use temporary field names to avoid conflicts during the renaming process.
• Dot notation creates nested object structures automatically.
• updateMany() with empty filter {} updates all documents in the collection.

Conclusion

The $rename operator with temporary fields effectively transforms simple fields into nested objects. This two-step approach ensures clean restructuring of document schemas in MongoDB collections.