Schema versioning strategies in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of schema version check query

Given a MongoDB collection users where each document has a schemaVersion field, what is the output of this query?

db.users.find({ schemaVersion: { $lt: 3 } }).count()

Assuming the collection has 5 documents with schemaVersion values: 1, 2, 3, 4, 5.

MongoDB

db.users.find({ schemaVersion: { $lt: 3 } }).count()

A 3

B 5

C 2

D 0

💡 Hint

Count documents where schemaVersion is less than 3.

Documents with schemaVersion 1 and 2 match the condition, so count is 2.

🧠 Conceptual

intermediate

Choosing a schema versioning strategy

Which schema versioning strategy is best when you want to keep old and new document formats in the same MongoDB collection and handle them differently in your application?

A Add a <code>schemaVersion</code> field and write application logic to handle each version

B Create a new collection for each schema version

C Overwrite old documents with new schema without versioning

D Use MongoDB transactions to lock schema changes

💡 Hint

Think about how to identify document versions inside the same collection.

Adding a schemaVersion field allows the application to detect and handle different document formats in one collection.

🔧 Debug

advanced

Identify the error in schema migration script

This MongoDB migration script aims to update all documents from schemaVersion 1 to 2 by adding a new field status with value active. What error will this script cause?

db.users.updateMany(
  { schemaVersion: 1 },
  { $set: { status: 'active' }, $inc: { schemaVersion: 1 } }
)

A Update does not change <code>schemaVersion</code> field

B No error, updates documents correctly

C Runtime error because <code>schemaVersion</code> is not numeric

D SyntaxError due to incorrect update operators usage

💡 Hint

Check if multiple update operators are combined correctly in one update document.

No error, the script correctly uses multiple update operators ($set and $inc) at the top level of the update document.

❓ optimization

advanced

Optimizing schema version queries

You have a large MongoDB collection with mixed schema versions. You often query documents with schemaVersion 3. Which index will optimize these queries best?

A { schemaVersion: 1 }

B { status: 1 }

C { schemaVersion: 1, createdAt: -1 }

D { createdAt: -1 }

💡 Hint

Index fields used in query filters for faster lookups.

An index on schemaVersion speeds up queries filtering by that field.

🧠 Conceptual

expert

Handling schema evolution with minimal downtime

What is the best approach to evolve MongoDB document schemas in a live system with zero downtime and backward compatibility?

A Drop the collection and recreate it with the new schema

B Disable writes during schema migration

C Update all documents in a single blocking migration operation

D Use a schemaVersion field and write application code to support multiple versions simultaneously

💡 Hint

Consider how to keep the system running while supporting old and new data formats.

Using a schemaVersion field allows the application to read and write documents in different formats without downtime.