$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.

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