$unset operator for removing fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after using $unset to remove a field?

Consider a MongoDB collection named users with a document:

{ "_id": 1, "name": "Alice", "age": 30, "city": "New York" }

If you run the update:

db.users.updateOne({ _id: 1 }, { $unset: { age: "" } })

What will the document look like after this update?

A { "_id": 1, "name": "Alice", "age": "", "city": "New York" }

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

C { "_id": 1, "name": "Alice", "age": null, "city": "New York" }

D { "_id": 1, "name": "Alice", "city": "New York" }

💡 Hint

The $unset operator removes the specified field from the document.

Using $unset with a field name removes that field entirely from the document. The value provided (empty string here) is ignored.

🧠 Conceptual

intermediate

What does the $unset operator do in MongoDB?

Choose the best description of the $unset operator in MongoDB.

A It sets the value of a field to null.

B It removes the specified field from a document.

C It renames a field in a document.

D It adds a new field with a specified value.

💡 Hint

Think about what happens to a field when you want it gone.

The $unset operator deletes the specified field from the document, not just setting it to null or changing its name.

📝 Syntax

advanced

Which update command correctly removes multiple fields using $unset?

You want to remove the fields email and phone from a document with { _id: 5 }. Which command is correct?

A db.contacts.updateOne({ _id: 5 }, { $unset: { email: "", phone: "" } })

B db.contacts.updateOne({ _id: 5 }, { $unset: ["email", "phone"] })

C db.contacts.updateOne({ _id: 5 }, { $unset: { email: null, phone: null } })

D db.contacts.updateOne({ _id: 5 }, { $unset: { email: 0, phone: 0 } })

💡 Hint

The $unset operator expects an object with field names as keys.

The $unset operator requires an object where keys are field names and values are ignored but usually set to an empty string. Using an array or null/0 values is invalid syntax.

🔧 Debug

advanced

Why does this $unset update fail?

Given the command:

db.products.updateOne({ _id: 10 }, { $unset: { 'details.price' } })

The update fails with a syntax error. What is the cause?

A The $unset value for each field must be specified, even if empty.

B updateOne requires a third argument for $unset to work.

C Field names cannot contain dots in $unset.

D The filter {_id: 10} is invalid for $unset.

💡 Hint

Check the syntax of the $unset object carefully.

In $unset, each field must have a value (usually an empty string). Writing { $unset: { 'details.price' } } is invalid because the value is missing.

❓ optimization

expert

Efficiently removing multiple fields from many documents

You have a large collection orders with millions of documents. You want to remove the fields tempData and oldStatus from all documents. Which approach is best for performance?

A Drop the collection and recreate it without those fields.

B Run multiple updateOne commands for each document to remove one field at a time.

C Run a single updateMany with { $unset: { tempData: "", oldStatus: "" } } without a filter.

D Use find() to get all documents, then update each document individually in the application.

💡 Hint

Think about minimizing the number of operations sent to the database.

Using updateMany with $unset on multiple fields in one command is efficient and fast. Updating documents one by one or fetching them to update in the app is slow. Dropping the collection loses all data.