$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.

Practice

1. What does the $rename operator do in MongoDB?

easy

A. It changes the value of a field in documents.

B. It deletes a field from documents.

C. It changes the name of a field in documents.

D. It adds a new field to documents.

Solution

1. Step 1: Understand the purpose of $rename

   The $rename operator is used to change the name of an existing field in MongoDB documents.

2. Step 2: Compare with other update operators

   Unlike $unset which deletes fields, or $set which changes values or adds fields, $rename only changes field names.

3. Final Answer:

   It changes the name of a field in documents. -> Option C

4. Quick Check:

   $rename changes field names [OK]

Hint: Remember: $rename changes field names, not values or deletion [OK]

Common Mistakes:

• Confusing $rename with $set or $unset
• Thinking $rename changes field values
• Assuming $rename adds new fields

2. Which of the following is the correct syntax to rename the field oldName to newName in all documents of a collection named users?

easy

A. db.users.updateMany({}, { $rename: { oldName: newName } })

B. db.users.updateMany({}, { $rename: { oldName: 'newName' } })

C. db.users.updateMany({}, { $rename: { 'oldName': newName } })

D. db.users.updateMany({}, { $rename: { 'oldName': 'newName' } })

Solution

1. Step 1: Check the syntax for $rename

   The $rename operator requires a mapping object where both old and new field names are strings, so keys and values must be quoted.

2. Step 2: Validate each option

   db.users.updateMany({}, { $rename: { 'oldName': 'newName' } }) correctly uses quotes around both 'oldName' and 'newName'. Options A, C, and D miss quotes on either key or value, causing syntax errors.

3. Final Answer:

   db.users.updateMany({}, { $rename: { 'oldName': 'newName' } }) -> Option D

4. Quick Check:

   Field names in $rename must be strings [OK]

Hint: Always quote both old and new field names in $rename [OK]

Common Mistakes:

• Not quoting field names as strings
• Using variables instead of strings
• Mixing quotes and no quotes

3. Given the collection products with documents:

{ "_id": 1, "name": "Pen", "cost": 5 }

What will be the document after running:

db.products.updateOne({ _id: 1 }, { $rename: { "cost": "price" } })

and then fetching the document with db.products.findOne({ _id: 1 })?

medium

A. { "_id": 1, "name": "Pen", "price": 5 }

B. { "_id": 1, "name": "Pen", "cost": 5, "price": 5 }

C. { "_id": 1, "name": "Pen" }

D. { "_id": 1, "name": "Pen", "cost": 5 }

Solution

1. Step 1: Understand what $rename does in updateOne

   The $rename operator changes the field name from cost to price in the matched document.

2. Step 2: Predict the document after update

   The field cost is removed and replaced by price with the same value 5. Other fields remain unchanged.

3. Final Answer:

   { "_id": 1, "name": "Pen", "price": 5 } -> Option A

4. Quick Check:

   $rename moves field value to new name [OK]

Hint: Rename moves value, old field disappears [OK]

Common Mistakes:

• Expecting both old and new fields to exist
• Thinking $rename changes field values
• Assuming $rename deletes other fields

4. You run the following command:

db.orders.updateMany({}, { $rename: { "orderDate": "date" } })

but get an error. What is the most likely cause?

medium

A. The collection orders does not exist.

B. The new field name "date" already exists in some documents.

C. Field names in $rename must be quoted strings.

D. The updateMany command requires a filter other than {}.

Solution

1. Step 1: Analyze the error cause for $rename

   If the new field name already exists in documents, MongoDB throws an error because it cannot rename a field to an existing field name.

2. Step 2: Check other options

   Field names in $rename must be quoted strings. is incorrect because field names are quoted correctly. The collection orders does not exist. is unlikely as the error is specific to $rename. The updateMany command requires a filter other than {}. is invalid because {} is a valid filter for updateMany.

3. Final Answer:

   The new field name "date" already exists in some documents. -> Option B

4. Quick Check:

   $rename fails if new field exists [OK]

Hint: Cannot rename to existing field name [OK]

Common Mistakes:

• Ignoring existing fields with new name
• Assuming empty filter is invalid
• Confusing syntax errors with runtime errors

5. You have a collection employees with documents containing fields fname and lname. You want to rename fname to firstName and lname to lastName in one update command. Which command correctly does this?

hard

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

B. db.employees.updateMany({}, { $rename: { 'fname': 'firstName' }, { 'lname': 'lastName' } })

C. db.employees.updateMany({}, { $rename: { fname: 'firstName', lname: 'lastName' } })

D. db.employees.updateMany({}, { $rename: { 'fname': firstName, 'lname': lastName } })

Solution

1. Step 1: Understand $rename supports multiple fields

   The $rename operator accepts an object mapping old field names to new field names, allowing multiple renames in one command.

2. Step 2: Check syntax correctness

   db.employees.updateMany({}, { $rename: { 'fname': 'firstName', 'lname': 'lastName' } }) correctly uses a single object with quoted old and new field names. db.employees.updateMany({}, { $rename: { 'fname': 'firstName' }, { 'lname': 'lastName' } }) has invalid syntax with two separate objects. db.employees.updateMany({}, { $rename: { fname: 'firstName', lname: 'lastName' } }) misses quotes on keys. db.employees.updateMany({}, { $rename: { 'fname': firstName, 'lname': lastName } }) misses quotes on new field names.

3. Final Answer:

   db.employees.updateMany({}, { $rename: { 'fname': 'firstName', 'lname': 'lastName' } }) -> Option A

4. Quick Check:

   Multiple renames use one $rename object with string keys and values [OK]

Hint: Use one $rename object with all old:new pairs quoted [OK]

Common Mistakes:

• Trying multiple $rename objects in one update
• Not quoting new field names
• Using variables instead of strings