$set operator for setting fields in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $set operator do in MongoDB?

The $set operator updates the value of a field in a document. If the field does not exist, it adds the field with the specified value.

beginner

How do you use $set to change the value of a field named status to active?

You use { $set: { status: 'active' } } inside an update command to set the status field to active.

beginner

Can $set add new fields to a document if they don't exist?

Yes, $set adds the field with the given value if the field is not already in the document.

beginner

What happens if you use $set on a field that already has a value?

The existing value of the field is replaced with the new value provided in the $set operation.

intermediate

Write a MongoDB update command using $set to change the age field to 30 for documents where name is 'Alice'.

db.collection.updateMany({ name: 'Alice' }, { $set: { age: 30 } })

What does the $set operator do in MongoDB?

A Increments a numeric field by 1

B Deletes a field from a document

C Updates or adds a field with a specified value

D Replaces the entire document

$set updates the value of a field or adds it if it doesn't exist.

If you want to add a new field city with value Paris to a document, which operator do you use?

A $set

B $unset

C $push

D $inc

$set adds new fields or updates existing ones.

What happens if $set is used on a field that already exists?

A The operation fails

B The field is removed

C The field value is incremented

D The field value is replaced

$set replaces the existing field value with the new one.

Which of these is a correct use of $set to update a field score to 100?

A { $push: { score: 100 } }

B { $set: { score: 100 } }

C { $unset: { score: 100 } }

D { $inc: { score: 100 } }

$set sets the field to the specified value.

Which MongoDB command updates all documents with status 'pending' to have status 'complete'?

A db.collection.updateMany({ status: 'pending' }, { $set: { status: 'complete' } })

B db.collection.updateMany({ status: 'pending' }, { $unset: { status: 'complete' } })

C db.collection.updateMany({ status: 'complete' }, { $set: { status: 'pending' } })

D db.collection.updateMany({ status: 'pending' }, { $inc: { status: 1 } })

This command sets the status field to 'complete' for all matching documents.

Explain how the $set operator works in MongoDB updates.

Think about changing or adding information in a document.

Describe a real-life example where you would use $set in a MongoDB database.

Imagine changing details in a contact list or order system.

Practice

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

easy

A. It updates the value of specified fields or adds them if they don't exist.

B. It deletes specified fields from a document.

C. It replaces the entire document with a new one.

D. It creates a new collection in the database.

Solution

1. Step 1: Understand the purpose of $set

   The $set operator is used to update existing fields or add new fields in a MongoDB document without affecting other fields.

2. Step 2: Compare with other operations

   Deleting fields uses $unset, replacing documents uses replaceOne, and creating collections is unrelated to $set.

3. Final Answer:

   It updates the value of specified fields or adds them if they don't exist. -> Option A

4. Quick Check:

   $set updates or adds fields [OK]

Hint: Remember: $set changes or adds fields only [OK]

Common Mistakes:

• Confusing $set with $unset which deletes fields
• Thinking $set replaces the whole document
• Assuming $set creates collections

2. Which of the following is the correct syntax to set the field age to 30 in a document using $set?

easy

A. db.collection.updateOne({name: 'John'}, {$set: [age: 30]})

B. db.collection.updateOne({name: 'John'}, {$set: {age: 30}})

C. db.collection.updateOne({name: 'John'}, {$set: 'age': 30})

D. db.collection.updateOne({name: 'John'}, {$set: age = 30})

Solution

1. Step 1: Identify correct $set syntax

   The $set operator requires an object with field-value pairs inside curly braces: {$set: {field: value}}.

2. Step 2: Check each option

   db.collection.updateOne({name: 'John'}, {$set: {age: 30}}) uses correct syntax with {age: 30}. Options A, B, and C use invalid JavaScript or MongoDB syntax.

3. Final Answer:

   db.collection.updateOne({name: 'John'}, {$set: {age: 30}}) -> Option B

4. Quick Check:

   Correct $set syntax uses object with field-value pairs [OK]

Hint: Use {$set: {field: value}} with curly braces [OK]

Common Mistakes:

• Using assignment (=) inside $set
• Using quotes incorrectly around field names
• Using array brackets instead of object braces

3. Given the document { name: 'Alice', city: 'Paris' }, what will be the document after running db.users.updateOne({name: 'Alice'}, {$set: {city: 'London', age: 25}})?

medium

A. { city: 'London', age: 25 }

B. { name: 'Alice', city: 'Paris', age: 25 }

C. { name: 'Alice', city: 'London', age: 25 }

D. { name: 'Alice', city: 'London' }

Solution

1. Step 1: Understand $set effect on fields

   The $set operator updates existing fields and adds new fields without removing others.

2. Step 2: Apply update to the document

   Field city changes from 'Paris' to 'London', and new field age with value 25 is added. The name field remains unchanged.

3. Final Answer:

   { name: 'Alice', city: 'London', age: 25 } -> Option C

4. Quick Check:

   $set updates and adds fields, keeps others [OK]

Hint: Only fields in $set change; others stay same [OK]

Common Mistakes:

• Assuming $set removes fields not listed
• Thinking $set only adds but does not update
• Confusing updateOne with replaceOne

4. You run this update: db.products.updateOne({id: 101}, {$set: {price: 20, stock}}). What is the problem?

medium

A. Missing value for the field 'stock' inside $set causes a syntax error.

B. The query filter {id: 101} is invalid.

C. You cannot update multiple fields with $set.

D. The update will delete the document.

Solution

1. Step 1: Analyze the $set object

   The $set operator requires each field to have a value. Here, stock is listed without a value, which is invalid syntax.

2. Step 2: Check other parts of the update

   The filter {id: 101} is valid, and $set can update multiple fields. The update does not delete documents.

3. Final Answer:

   Missing value for the field 'stock' inside $set causes a syntax error. -> Option A

4. Quick Check:

   Each field in $set must have a value [OK]

Hint: Every field in $set needs a value [OK]

Common Mistakes:

• Leaving out values for fields in $set
• Confusing filter syntax with update syntax
• Thinking $set deletes documents

5. You want to update all documents in the employees collection to add a new field status with value 'active' only if the field status does not already exist. Which update command using $set is correct?

hard

A. db.employees.updateMany({}, {$set: {status: 'active'}})

B. db.employees.updateMany({status: {$ne: 'active'}}, {$set: {status: 'active'}})

C. db.employees.updateMany({status: null}, {$set: {status: 'active'}})

D. db.employees.updateMany({status: {$exists: false}}, {$set: {status: 'active'}})

Solution

1. Step 1: Identify condition to update only missing 'status'

   The filter {status: {$exists: false}} selects documents where the 'status' field does not exist.

2. Step 2: Use $set to add 'status' field

   The update {$set: {status: 'active'}} adds the 'status' field with value 'active' only to those documents.

3. Step 3: Check other options

   db.employees.updateMany({}, {$set: {status: 'active'}}) updates all documents regardless of existing 'status'. db.employees.updateMany({status: null}, {$set: {status: 'active'}}) only matches documents with 'status' equal to null, not missing. db.employees.updateMany({status: {$ne: 'active'}}, {$set: {status: 'active'}}) updates documents where 'status' is not 'active', which may overwrite existing values.

4. Final Answer:

   db.employees.updateMany({status: {$exists: false}}, {$set: {status: 'active'}}) -> Option D

5. Quick Check:

   Use {$exists: false} to target missing fields [OK]

Hint: Filter with {$exists: false} to add missing fields only [OK]

Common Mistakes:

• Updating all documents without filter
• Using null instead of $exists for missing fields
• Overwriting existing 'status' values unintentionally