$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".