$unset operator for removing fields in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove the field 'age' from all documents.

MongoDB

db.collection.updateMany({}, [1])

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

A{ $set: { age: null } }

B{ $remove: { age: 1 } }

C{ $unset: { age: "" } }

D{ $delete: { age: true } }

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set to null instead of $unset to remove the field.

Using non-existent operators like $remove or $delete.

The $unset operator removes the specified field from documents. Using { $unset: { age: "" } } removes the 'age' field.

2fill in blank

medium

Complete the code to remove the 'address' field from documents where 'status' is 'inactive'.

MongoDB

db.collection.updateMany({ status: "inactive" }, [1])

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

A{ $remove: { address: 1 } }

B{ $unset: { status: "" } }

C{ $set: { address: null } }

D{ $unset: { address: "" } }

Attempts:

3 left

💡 Hint

Common Mistakes

Removing the wrong field.

Using $set instead of $unset.

To remove the 'address' field only from documents with status: 'inactive', use $unset with the field name.

3fill in blank

hard

Fix the error in the code to correctly remove the 'phone' field from all documents.

MongoDB

db.collection.updateMany({}, { $unset: { phone: [1] } })

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

A""

Bnull

Ctrue

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or true instead of empty string.

Using numeric values like 1 which do not work with $unset.

The $unset operator requires the field to be set to an empty string "" to remove it.

4fill in blank

hard

Fill both blanks to remove 'email' and 'username' fields from documents where 'active' is false.

MongoDB

db.collection.updateMany({ active: false }, { $unset: { [1]: "", [2]: "" } })

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

Aemail

Busername

Cactive

Dpassword

Attempts:

3 left

💡 Hint

Common Mistakes

Removing wrong fields like 'active' or 'password'.

Not using empty strings as values.

To remove multiple fields, list each field with an empty string value inside $unset.

5fill in blank

hard

Fill all three blanks to remove 'city', 'state', and 'zip' fields from documents where 'country' is 'USA'.

MongoDB

db.collection.updateMany({ country: "USA" }, { $unset: { [1]: "", [2]: "", [3]: "" } })

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

Acity

Bstate

Czip

Dcountry

Attempts:

3 left

💡 Hint

Common Mistakes

Including 'country' in the fields to remove.

Using non-empty values for $unset fields.

To remove multiple fields, list each field with an empty string value inside $unset. Here, 'city', 'state', and 'zip' are removed.

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