$unset operator for removing fields in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $unset operator do in MongoDB?

The $unset operator removes a specified field from a document in a MongoDB collection.

beginner

How do you use $unset to remove the field age from a document?

Use { $unset: { age: "" } } in an update operation to remove the age field.

intermediate

Can $unset remove multiple fields at once? How?

Yes, by listing multiple fields inside $unset, like { $unset: { field1: "", field2: "" } }.

intermediate

What happens if you try to $unset a field that does not exist?

Nothing changes; MongoDB ignores the $unset for fields that are not present.

beginner

Write a MongoDB update command to remove the field address from all documents in the collection users.

db.users.updateMany({}, { $unset: { address: "" } })

This removes the address field from every document.

What value should you assign to a field inside $unset to remove it?

A true

B An empty string ""

C null

D 0

The $unset operator uses an empty string "" as the value to indicate the field should be removed.

Which MongoDB method is commonly used with $unset to update multiple documents?

A updateMany()

B find()

C insertMany()

D updateOne()

updateMany() updates all documents matching the filter and can be used with $unset to remove fields.

If a field does not exist in a document, what does $unset do?

A Ignores and makes no change

B Removes the document

C Throws an error

D Adds the field with null value

$unset ignores fields that do not exist and does not cause errors.

How do you remove two fields, phone and email, at once using $unset?

A { $unset: { phone: null, email: null } }

B { $unset: ["phone", "email"] }

C { $unset: { phone: "", email: "" } }

D { $unset: { phone: true, email: true } }

You list each field with an empty string value inside $unset to remove multiple fields.

Which of these is a valid use of $unset in an update command?

A db.collection.find({ $unset: { age: "" } })

B db.collection.deleteOne({ $unset: { age: "" } })

C db.collection.insertOne({ $unset: { age: "" } })

D db.collection.updateOne({}, { $unset: { age: "" } })

$unset is used in update commands like updateOne() to remove fields.

Explain how the $unset operator works in MongoDB and give an example of removing a field.

Think about how you delete a property from an object in real life.

Describe what happens if you use $unset on a field that does not exist in a document.

Imagine trying to remove a drawer that is already empty.