$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.

Practice

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

easy

A. Updates the value of existing fields

B. Adds new fields to documents

C. Removes specified fields from documents

D. Duplicates documents in a collection

Solution

1. Step 1: Understand the purpose of $unset

   The $unset operator is used to remove fields from documents in MongoDB.

2. Step 2: Compare with other operators

   Unlike $set which adds or updates fields, $unset deletes them.

3. Final Answer:

   Removes specified fields from documents -> Option C

4. Quick Check:

   $unset removes fields [OK]

Hint: Remember: $unset deletes fields, $set adds or updates [OK]

Common Mistakes:

• Confusing $unset with $set
• Thinking $unset adds fields
• Assuming $unset duplicates documents

2. Which of the following is the correct syntax to remove the field age from all documents in a collection named users?

easy

A. db.users.updateMany({}, {$unset: {age: null}})

B. db.users.updateMany({}, {$unset: {age: true}})

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

D. db.users.updateMany({}, {$unset: {age: 1}})

Solution

1. Step 1: Recall correct $unset syntax

   The $unset operator requires the field name with a value of 1 to remove it.

2. Step 2: Check each option

   Only db.users.updateMany({}, {$unset: {age: 1}}) uses age: 1, which is the correct way to remove the field.

3. Final Answer:

   db.users.updateMany({}, {$unset: {age: 1}}) -> Option D

4. Quick Check:

   Use 1 as value with $unset [OK]

Hint: Use 1 as value to remove fields with $unset [OK]

Common Mistakes:

• Using true, null, or empty string instead of 1
• Forgetting curly braces around field
• Using $set instead of $unset

3. Given the collection products with documents:

{ "name": "Pen", "price": 5, "color": "blue" }

{ "name": "Notebook", "price": 10, "color": "red" }

What will be the result of this update?

db.products.updateMany({}, { $unset: { color: 1 } })

What will the documents look like after the update?

medium

A. [{ "name": "Pen", "price": 5, "color": "blue" }, { "name": "Notebook", "price": 10, "color": "red" }]

B. [{ "name": "Pen", "price": 5 }, { "name": "Notebook", "price": 10 }]

C. [{ "name": "Pen", "color": "blue" }, { "name": "Notebook", "color": "red" }]

D. [{ "price": 5, "color": "blue" }, { "price": 10, "color": "red" }]

Solution

1. Step 1: Understand $unset effect

   The $unset operator removes the specified field from all documents.

2. Step 2: Apply $unset to color field

   Removing color means it will no longer appear in any document, leaving only name and price.

3. Final Answer:

   [{ "name": "Pen", "price": 5 }, { "name": "Notebook", "price": 10 }] -> Option B

4. Quick Check:

   Fields removed by $unset disappear from documents [OK]

Hint: Fields removed by $unset vanish from all matched documents [OK]

Common Mistakes:

• Expecting fields to be set to null instead of removed
• Thinking $unset changes field values
• Assuming only one document is affected

4. You run this command:

db.orders.updateMany({}, { $unset: { "status" } })

But it throws an error. What is the problem?

medium

A. The field name must be a string key with a value, not just a key

B. You cannot use $unset with updateMany

C. The collection name is incorrect

D. The $unset operator requires an array of fields

Solution

1. Step 1: Check $unset syntax

   The $unset operator requires field names as keys with a value (usually 1) to specify removal.

2. Step 2: Identify missing value

   In the command, { $unset: { "status" } } is invalid because the field "status" has no value.

3. Final Answer:

   The field name must be a string key with a value, not just a key -> Option A

4. Quick Check:

   $unset needs field: value pairs [OK]

Hint: Always provide a value (like 1) with field names in $unset [OK]

Common Mistakes:

• Omitting the value for the field in $unset
• Using $unset with wrong update method
• Assuming $unset accepts arrays

5. You have documents in employees collection with fields name, age, department, and tempField. You want to remove tempField only from employees in the Sales department without affecting others. Which command achieves this?

hard

A. db.employees.updateMany({ department: "Sales" }, { $unset: { tempField: 1 } })

B. db.employees.updateMany({}, { $unset: { tempField: 1 } })

C. db.employees.updateMany({ tempField: { $exists: true } }, { $unset: { department: 1 } })

D. db.employees.updateMany({ department: "Sales" }, { $set: { tempField: null } })

Solution

1. Step 1: Filter documents by department

   Use the query { department: "Sales" } to target only Sales employees.

2. Step 2: Use $unset to remove tempField

   Apply $unset: { tempField: 1 } to remove the field from matched documents only.

3. Final Answer:

   db.employees.updateMany({ department: "Sales" }, { $unset: { tempField: 1 } }) -> Option A

4. Quick Check:

   Filter then $unset removes fields selectively [OK]

Hint: Filter first, then $unset fields to remove selectively [OK]

Common Mistakes:

• Removing fields from all documents unintentionally
• Using $set to null instead of $unset
• Removing wrong fields by mistake