Question 1

What is MongoDB and how does it differ from relational databases?

Accepted Answer

MongoDB is a document-oriented NoSQL database storing BSON documents in collections. Unlike SQL tables with fixed schemas and joins, MongoDB uses flexible documents, favors embedding over joins, and scales horizontally with sharding.

Question 2

What is BSON?

Accepted Answer

BSON (Binary JSON) is MongoDB's storage format — a binary-encoded superset of JSON supporting additional types like Date, ObjectId, int64, and Decimal128.

Question 3

Explain the difference between find() and aggregate().

Accepted Answer

find() returns documents matching a filter as stored. aggregate() runs a pipeline of stages ($match, $group, $lookup) to filter, transform, join, and compute analytics in the database.

Question 4

When would you embed vs reference data?

Accepted Answer

Embed when data is accessed together and has one-to-few cardinality (order line items). Reference with ObjectId when data is large, shared across documents, or updated independently (user profile referenced by orders).

Question 5

What is the purpose of an index in MongoDB?

Accepted Answer

Indexes are B-tree structures that let queries locate documents without scanning the entire collection. They speed reads but add overhead on writes.

Question 6

What is the ESR rule for compound indexes?

Accepted Answer

Equality fields first, Sort fields second, Range fields last. Example: { status: 1, createdAt: -1, price: 1 } for filtering by status, sorting by date, and ranging on price.

Question 7

Difference between updateOne and replaceOne?

Accepted Answer

updateOne applies update operators ($set, $inc) to specific fields. replaceOne replaces the entire document (except _id) with the new document provided.

Question 8

What does $lookup do?

Accepted Answer

$lookup performs a left outer join — for each input document, it looks up matching documents in another collection and adds them to an array field.

Question 9

What is a replica set?

Accepted Answer

A replica set is a group of MongoDB servers maintaining the same data. One primary handles writes; secondaries replicate for redundancy and optional read scaling.

Question 10

Does MongoDB support ACID transactions?

Accepted Answer

Yes, since MongoDB 4.0, multi-document ACID transactions are supported on replica sets. Use sessions and withTransaction for operations that must succeed or fail together.

Question 11

What is NoSQL injection and how do you prevent it?

Accepted Answer

Attackers pass crafted objects as query filters (e.g. { $gt: '' }). Prevent by validating input types, using schema validation, never passing raw user objects to queries, and using parameterized driver methods.

Question 12

Explain $elemMatch.

Accepted Answer

$elemMatch matches array elements that satisfy multiple conditions on the same element. Example: items: { $elemMatch: { qty: { $gt: 5 }, price: { $lt: 100 } } }.

Question 13

What is the difference between deleteOne and drop()?

Accepted Answer

deleteOne/deleteMany remove matching documents but keep the collection and indexes. drop() removes the entire collection including its indexes.

Question 14

What is Mongoose?

Accepted Answer

Mongoose is an ODM for Node.js that defines schemas, validation, middleware hooks, and a model API on top of the MongoDB driver.

Question 15

What is a TTL index?

Accepted Answer

A TTL (Time To Live) index automatically deletes documents after a specified seconds-to-live on a date field — useful for sessions, logs, and cache entries.

Question 16

What is the maximum document size?

Accepted Answer

16 megabytes per document. Larger files use GridFS which splits files into chunks stored across documents.

Question 17

Difference between $push and $addToSet?

Accepted Answer

$push appends a value to an array. $addToSet adds only if the value is not already present — like a set.

Question 18

What does explain('executionStats') show?

Accepted Answer

It shows the query plan: whether IXSCAN or COLLSCAN was used, documents examined vs returned, execution time — essential for index tuning.

Question 19

What is sharding?

Accepted Answer

Sharding distributes data across multiple servers (shards) by a shard key. It enables horizontal scaling when a single replica set cannot hold or serve the dataset.

Question 20

What is a capped collection?

Accepted Answer

A fixed-size collection that automatically overwrites oldest documents when full — used for logs and queues with FIFO behavior.

MongoDB Interview Questions