$rename operator for field names in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $rename operator do in MongoDB?

The $rename operator changes the name of a field in a document without altering its value.

beginner

How do you use $rename to rename a field called oldName to newName?

Use { $rename: { 'oldName': 'newName' } } inside an update operation to rename the field.

intermediate

Can $rename rename multiple fields at once?

Yes, you can rename multiple fields by listing them all inside the $rename object, like { $rename: { 'old1': 'new1', 'old2': 'new2' } }.

intermediate

What happens if the new field name already exists when using $rename?

MongoDB will overwrite the existing field with the renamed field's value.

advanced

Is it possible to rename a nested field using $rename?

Yes, you can rename nested fields by specifying the full path with dot notation, like { $rename: { 'address.street': 'address.road' } }.

Which MongoDB operator is used to rename a field in a document?

A $unset

B $set

C $rename

D $push

The $rename operator changes the name of a field.

How do you rename the field username to user_name in MongoDB?

A { $rename: { 'username': 'user_name' } }

B { $unset: { 'username': '' } }

C { $set: { 'user_name': 'username' } }

D { $rename: { 'user_name': 'username' } }

The syntax is { $rename: { 'oldField': 'newField' } }.

What happens if the new field name already exists when using $rename?

A The operation fails with an error.

B The existing field is overwritten.

C Both fields are kept with the same name.

D The rename is ignored.

MongoDB overwrites the existing field with the renamed field's value.

Can $rename be used to rename nested fields?

A No, it only works on top-level fields.

B Only if the nested field is an array.

C Only if the nested field is a string.

D Yes, by using dot notation for the field path.

You rename nested fields by specifying the full path with dot notation.

Which of the following is a valid $rename update to rename two fields at once?

A { $rename: { 'a': 'b', 'c': 'd' } }

B { $rename: { 'a': 'b' }, { 'c': 'd' } }

C { $rename: ['a', 'b', 'c', 'd'] }

D { $rename: 'a to b, c to d' }

Multiple fields can be renamed by listing them inside the same $rename object.

Explain how to rename a field in a MongoDB document using the $rename operator.

Think about how you tell MongoDB which field to change and what to rename it to.

Describe what happens if the new field name already exists when using $rename.

Consider if MongoDB merges, errors, or replaces fields.

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