$set operator for setting fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this update operation?

Consider a MongoDB collection users with a document:

{ "_id": 1, "name": "Alice", "age": 25 }

What will be the document after running this update?

db.users.updateOne({ _id: 1 }, { $set: { age: 26, city: "New York" } })

A { "_id": 1, "name": "Alice", "age": 26, "city": "New York" }

B { "_id": 1, "name": "Alice", "age": 25, "city": "New York" }

C { "_id": 1, "name": "Alice", "age": 26 }

D { "_id": 1, "name": "Alice", "city": "New York" }

💡 Hint

The $set operator changes existing fields or adds new fields without removing others.

The $set operator updates the age from 25 to 26 and adds a new field city with value "New York". Other fields remain unchanged.

📝 Syntax

intermediate

Which update command uses $set correctly?

You want to update the field status to "active" in a MongoDB document. Which of these commands is syntactically correct?

A db.collection.updateOne({ _id: 1 }, { $set: ["status", "active"] })

B db.collection.updateOne({ _id: 1 }, { set: { status: "active" } })

C db.collection.updateOne({ _id: 1 }, { $set: { status: "active" } })

D db.collection.updateOne({ _id: 1 }, { $set: status = "active" })

💡 Hint

The $set operator must be followed by an object with field-value pairs.

Option C uses $set with an object correctly. Options A, B, and C have syntax errors or wrong usage.

❓ query_result

advanced

What is the output after this update with nested fields?

Given a document:

{ "_id": 2, "profile": { "name": "Bob", "age": 30 } }

What will the document look like after:

db.collection.updateOne({ _id: 2 }, { $set: { "profile.age": 31, "profile.city": "Boston" } })

A { "_id": 2, "profile": { "name": "Bob", "age": 30, "city": "Boston" } }

B { "_id": 2, "profile": { "name": "Bob", "age": 31, "city": "Boston" } }

C { "_id": 2, "profile": { "name": "Bob", "age": 31 }, "city": "Boston" }

D { "_id": 2, "profile": { "name": "Bob", "age": 30 }, "profile.city": "Boston" }

💡 Hint

Using dot notation in $set updates nested fields inside the object.

The $set with "profile.age" updates age to 31 inside profile. Adding "profile.city" adds city inside profile. So profile has name, age=31, city.

🔧 Debug

advanced

Why does this update fail?

You run this update:

db.collection.updateOne({ _id: 3 }, { $set: { age } })

But it throws an error. Why?

A Because the collection name is missing.

B Because the _id field cannot be used in update filters.

C Because updateOne requires $inc operator instead of $set for numbers.

D Because $set requires a field-value pair, but only a field name 'age' is given without a value.

💡 Hint

Check the syntax inside the $set object.

The $set operator expects an object with field names and their new values. Writing { age } alone is invalid syntax.

🧠 Conceptual

expert

What happens if you use $set with a field that is an array?

Given a document:

{ "_id": 4, "tags": ["red", "blue"] }

What will be the document after:

db.collection.updateOne({ _id: 4 }, { $set: { tags: "green" } })

A { "_id": 4, "tags": "green" }

B { "_id": 4, "tags": ["red", "blue", "green"] }

C { "_id": 4, "tags": ["green"] }

D The update will fail with a type error.

💡 Hint

The $set operator replaces the entire field value with the new value.

Using $set on an array field replaces the whole array with the new value. Here, tags becomes the string "green".

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