$rename operator for field names in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after using $rename to change a field name?

Given a MongoDB collection with a document:

{ "_id": 1, "name": "Alice", "age": 30 }

After running this update:

db.collection.updateOne({ _id: 1 }, { $rename: { "name": "fullName" } })

What does the document look like now?

A { "_id": 1, "name": "Alice", "fullName": "Alice", "age": 30 }

B { "_id": 1, "name": "Alice", "age": 30 }

C { "_id": 1, "fullName": "Alice", "age": 30 }

D { "_id": 1, "age": 30 }

💡 Hint

The $rename operator changes the name of a field without changing its value.

The $rename operator renames the field "name" to "fullName". The original "name" field is removed and replaced by "fullName" with the same value.

📝 Syntax

intermediate

Which $rename update query is syntactically correct?

Choose the valid MongoDB update query that renames the field "oldField" to "newField".

A db.collection.updateOne({}, { $rename: ["oldField", "newField"] })

B db.collection.updateOne({}, { $rename: { oldField: newField } })

C db.collection.updateOne({}, { rename: { "oldField": "newField" } })

D db.collection.updateOne({}, { $rename: { "oldField": "newField" } })

💡 Hint

Field names and new names must be strings inside the $rename object.

The $rename operator requires an object with string keys and string values representing old and new field names. Option D uses correct syntax with quotes.

🧠 Conceptual

advanced

What happens if you try to rename a field to an existing field name?

Suppose a document has fields "a" and "b". You run:

db.collection.updateOne({}, { $rename: { "a": "b" } })

What is the result?

A The update fails with an error because "b" already exists.

B The update succeeds but creates a nested field "b.a".

C Both fields "a" and "b" remain unchanged.

D The field "a" is renamed to "b", overwriting the original "b" field.

💡 Hint

MongoDB does not allow renaming a field to a name that already exists in the document.

Renaming a field to an existing field name causes a conflict and MongoDB throws an error to prevent data loss.

❓ optimization

advanced

How to rename multiple fields in one update efficiently?

You want to rename fields "firstName" to "fname" and "lastName" to "lname" in one update. Which query is best?

A db.collection.updateMany({}, { $rename: { "firstName": "fname", "lastName": "lname" } })

B db.collection.updateMany({}, { $rename: { "firstName": "fname" } }); db.collection.updateMany({}, { $rename: { "lastName": "lname" } })

C db.collection.updateMany({}, { $set: { "fname": "$firstName", "lname": "$lastName" }, $unset: ["firstName", "lastName"] })

D db.collection.updateMany({}, { $rename: ["firstName", "fname"], $rename: ["lastName", "lname"] })

💡 Hint

You can rename multiple fields in one $rename object.

Option A uses a single update with $rename specifying both fields, which is efficient and atomic. Option A runs two updates, less efficient. Option A is invalid syntax. Option A is invalid syntax.

🔧 Debug

expert

Why does this $rename update fail with a syntax error?

Consider this update query:

db.collection.updateOne({}, { $rename: { 'oldField': 'newField', } })

It causes a syntax error. What is the cause?

A Single quotes cannot be used for field names in MongoDB update operators.

B Trailing comma after the last key-value pair in the $rename object is invalid in MongoDB shell.

C The $rename operator requires an array, not an object.

D The updateOne method requires a filter with at least one field.

💡 Hint

Check the syntax of the JavaScript object inside the update.

In JavaScript, trailing commas in object literals are allowed in modern environments, but some MongoDB shells or drivers may reject them causing syntax errors.