$rename operator for field names in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to rename the field 'oldName' to 'newName' in a MongoDB update operation.

MongoDB

db.collection.updateOne({}, { $rename: { [1]: 'newName' } })

Drag options to blanks, or click blank then click option'

A'newName'

B'oldName'

C'old_name'

D'nameOld'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the new field name as the key instead of the old field name.

Forgetting to put the field name in quotes.

The $rename operator requires the current field name as the key and the new field name as the value.

2fill in blank

medium

Complete the code to rename the field 'username' to 'user_name' in all documents.

MongoDB

db.collection.updateMany({}, { $rename: { [1]: 'user_name' } })

Drag options to blanks, or click blank then click option'

A'username'

B'user_name'

C'userName'

D'user-name'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the new field name as the key.

Changing the case of the field name incorrectly.

The $rename operator needs the exact current field name as the key to rename it.

3fill in blank

hard

Fix the error in the code to rename 'address' to 'location' in one document.

MongoDB

db.collection.updateOne({}, { $rename: { address: [1] } })

Drag options to blanks, or click blank then click option'

Alocation

B'address'

Clocation'

D'location'

Attempts:

3 left

💡 Hint

Common Mistakes

Not putting the new field name in quotes.

Using unbalanced quotes.

The new field name must be a string inside quotes when used as the value in $rename.

4fill in blank

hard

Fill both blanks to rename 'phone' to 'contactNumber' in all documents.

MongoDB

db.collection.updateMany({}, { $rename: { [1]: [2] } })

Drag options to blanks, or click blank then click option'

A'phone'

B'contactNumber'

C'phoneNumber'

D'contact_number'

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping old and new field names.

Forgetting quotes around field names.

The $rename operator needs the old field name as the key and the new field name as the value, both as strings.

5fill in blank

hard

Fill all three blanks to rename 'email' to 'contactEmail' and 'dob' to 'dateOfBirth' in one update operation.

MongoDB

db.collection.updateMany({}, { $rename: { [1]: [2], [3]: 'dateOfBirth' } })

Drag options to blanks, or click blank then click option'

A'email'

B'contactEmail'

C'dob'

D'dateOfBirth'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up old and new field names.

Not quoting field names properly.

Use $rename with old field names as keys and new field names as values, all as strings.

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