MongoDB Interview

How would you explain MongoDB to someone who already knows relational databases?

3 min·FundamentalsOpen

MongoDB stores documents. How is BSON different from JSON, and why does that matter?

3 min·FundamentalsOpen

MongoDB is schema-flexible. When does that help, and when does it hurt?

4 min·FundamentalsOpen

What is ObjectId, and when would you not use one as _id?

3 min·FundamentalsOpen

What does it mean that a single-document write in MongoDB is atomic?

4 min·FundamentalsOpen

What is the difference between find() and findOne()?

3 min·CRUDOpen

Why bother with projections if you are going to drop fields in application code anyway?

3 min·CRUDOpen

A user hits 'save preferences' twice in a row. How would you write the update so the document is created if it does not exist, without inserting duplicates?

4 min·CRUDOpen

When would you use $in rather than $or?

3 min·QueryingOpen

This query returns nothing, even though the user exists. What is wrong, and how would you fix it?

4 min·QueryingOpen

This user search is fine in development and unusable in production. What would you investigate?

4 min·QueryingOpen

A 'list users' API became slow and memory-heavy after you added profile fields, avatars, and a preferences object. The query is still find({ city }). What went wrong?

4 min·QueryingOpen

How do you query a nested field, and when must you use $elemMatch instead of dot notation on an array of objects?

4 min·ArraysOpen

You stored comments as an array on each post. The write path starts failing for popular posts. What happened, and what would you change?

4 min·ArraysOpen

This update is supposed to mark one skill as verified. What is wrong with it?

4 min·ArraysOpen

You need users who have both Node.js and MongoDB in a skills string array. How do you write that, and how is it different from $in?

4 min·ArraysOpen

What is the aggregation pipeline, and why does stage order matter?

4 min·AggregationOpen

How does $group work, and what does _id mean inside it?

4 min·AggregationOpen

What does $lookup do, and why is it sometimes expensive?

4 min·AggregationOpen

What does $unwind do, and what silent behaviour should you know about?

4 min·AggregationOpen

This pipeline was acceptable at 100k orders and is now painful at 50 million. What would you investigate, in order?

5 min·AggregationOpen

Product wants the top 3 customers by completed-order spend. How would you write the pipeline, and what index would you want?

4 min·AggregationOpen

An API always $lookups the user onto every order. Would you keep the join, embed the user snapshot, or duplicate a few fields? What factors decide?

5 min·AggregationOpen

A dashboard needs completed-order count, revenue, and top 5 products from the same day's orders. Would you use $facet or three aggregations?

4 min·AggregationOpen

How do you decide between embedding and referencing in MongoDB?

5 min·Data ModelingOpen

Would you embed order items inside an order or store them in a separate collection? What factors would you consider?

4 min·Data ModelingOpen

Would you embed product information inside an order line, or only store productId and $lookup the catalogue?

5 min·Data ModelingOpen

Users can enrol in many courses, and courses have many users. How would you model that in MongoDB?

4 min·Data ModelingOpen

How would you decide between MongoDB and PostgreSQL for a new backend service?

5 min·Data ModelingOpen

A chat thread is stored as one document with a messages array. What problems appear at scale, and what pattern would you use instead?

4 min·Data ModelingOpen

How do indexes work in MongoDB, and why would you not index every field?

4 min·IndexingOpen

How does field order in a compound index affect which queries can use it?

5 min·IndexingOpen

What is the ESR guideline for compound indexes, and why does the sort field go before range?

4 min·IndexingOpen

How would you prevent two users from registering with the same email?

3 min·IndexingOpen

Writes to orders slowed down after the team added an index for every dashboard filter. What happened, and how would you respond?

4 min·IndexingOpen

What is a covered query?

5 min·IndexingOpen

A query is slow. How do you diagnose it?

4 min·PerformanceOpen

You have 50 million orders. How would you efficiently fetch the latest 20 orders for a user?

5 min·PerformanceOpen

Page 1 of this API is fast. Page 5000 is not. Why, and what would you do instead?

4 min·PerformanceOpen

Is a COLLSCAN always a problem?

4 min·PerformanceOpen

Does MongoDB support transactions, and when do you actually need them?

5 min·TransactionsOpen

You must move money from account A to account B. Would you use a MongoDB transaction? What else would you consider?

5 min·TransactionsOpen

Two checkouts can both buy the last Mechanical Keyboard. What is wrong, and how do you fix it?

4 min·ConcurrencyOpen

Two requests credit the same wallet document at the same time using find, add in JS, then $set balance. How do you make this safe?

5 min·ConcurrencyOpen

A Node.js API creates new MongoClient() inside the request handler and closes it at the end. Latency is high and the cluster shows connection spikes. What is wrong?

4 min·Node.jsOpen

This query became slow after orders reached 50 million documents. What would you investigate?

4 min·Node.jsOpen

How should a Node.js web app share MongoDB connections across requests, and what goes in environment variables?

4 min·Node.jsOpen

What does lean() do in Mongoose, and when would you use it?

4 min·MongooseOpen

An order list API uses populate('user') and populate('items.product') and is slow. What is going on, and what would you change?

4 min·MongooseOpen

What is a replica set, and what do write concern and read preference control?

4 min·ProductionOpen