Single document atomicity in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the final value of the field after this update?

Consider a MongoDB document: { _id: 1, count: 5 }. You run this update operation:

db.collection.updateOne({ _id: 1 }, { $inc: { count: 3 } })

What will be the value of count after the update?

A 5

B 3

C 15

D 8

💡 Hint

The $inc operator adds the specified value to the existing field value.

The $inc operator increases the current value of 'count' by 3. Since it started at 5, the new value is 8.

🧠 Conceptual

intermediate

Why is a single document update atomic in MongoDB?

MongoDB guarantees that updates to a single document are atomic. What is the main reason for this guarantee?

A Because MongoDB applies the update as a single operation on the document

B Because MongoDB replicates the update to all nodes before confirming

C Because MongoDB uses transactions for every update

D Because MongoDB locks the entire database during the update

💡 Hint

Think about how MongoDB treats each document update internally.

MongoDB treats each update to a single document as one atomic operation, so it either fully applies or does not apply at all.

📝 Syntax

advanced

Which update query correctly increments a nested field atomically?

Given a document:

{ _id: 1, stats: { views: 10 } }

Which update query will correctly increment the views field inside stats by 5?

A db.collection.updateOne({ _id: 1 }, { $inc: { stats.views: 5 } })

B db.collection.updateOne({ _id: 1 }, { $inc: { stats->views: 5 } })

C db.collection.updateOne({ _id: 1 }, { $inc: { 'stats.views': 5 } })

D db.collection.updateOne({ _id: 1 }, { $inc: { stats['views']: 5 } })

💡 Hint

Use quotes around the dotted path to nested fields.

To update nested fields, the field name must be a string with dot notation inside quotes.

🔧 Debug

advanced

Why does this update fail to increment the field?

You run this update:

db.collection.updateOne({ _id: 1 }, { count: 10 })

But the count field does not increment, it replaces the entire document except _id. Why?

A Because the filter {_id: 1} is incorrect

B Because the update document is missing an update operator like $inc or $set

C Because MongoDB does not allow updates on numeric fields

D Because the collection is read-only

💡 Hint

Updates without operators replace the whole document except _id.

Without an update operator like $inc or $set, MongoDB treats the update as a replacement document.

❓ optimization

expert

How to ensure atomic increment of a field when multiple clients update concurrently?

Multiple clients try to increment the same field score in a document concurrently. Which approach ensures the increments are atomic and no updates are lost?

A Use the $inc operator in update queries without additional locking

B Read the current value, increment in application code, then update with $set

C Use transactions with multiple update statements to increment the field

D Lock the document manually before updating

💡 Hint

MongoDB's $inc operator is atomic on single documents.

The $inc operator increments the field atomically even with concurrent updates, preventing lost updates.