100% Free
🎯 3200+ Questions. One Goal: Your Success Sign In Sign Up

Entry MongoDB Interview Questions

Curated Entry-level MongoDB interview questions for developers targeting entry positions. 20 questions available.

Last updated:

MongoDB Interview Questions & Answers

Skip to Questions

Welcome to our comprehensive collection of MongoDB interview questions and answers. This page contains expertly curated interview questions covering all aspects of MongoDB, from fundamental concepts to advanced topics. Whether you're preparing for an entry-level position or a senior role, you'll find questions tailored to your experience level.

Our MongoDB interview questions are designed to help you:

  • Understand core concepts and best practices in MongoDB
  • Prepare for technical interviews at all experience levels
  • Master both theoretical knowledge and practical application
  • Build confidence for your next MongoDB interview

Each question includes detailed answers and explanations to help you understand not just what the answer is, but why it's correct. We cover topics ranging from basic MongoDB concepts to advanced scenarios that you might encounter in senior-level interviews.

Use the filters below to find questions by difficulty level (Entry, Junior, Mid, Senior, Expert) or focus specifically on code challenges. Each question is carefully crafted to reflect real-world interview scenarios you'll encounter at top tech companies, startups, and MNCs.

Questions

20 questions
Q1:

What is MongoDB?

Entry

Answer

MongoDB is a NoSQL, document-oriented database that stores data as JSON-like documents. It is schema-flexible and designed for scalable modern applications.
Quick Summary: MongoDB is a NoSQL document database that stores data as JSON-like documents (BSON). Unlike relational databases, there are no tables with fixed schemas - each document can have different fields. It's designed for flexibility, horizontal scaling, and developer productivity. Used widely for catalogs, user profiles, content, and real-time analytics.
Permalink
Q2:

What is a document in MongoDB?

Entry

Answer

A document is a JSON-like object containing key-value pairs. It is the basic unit of data stored inside collections in MongoDB.
Quick Summary: A document is a single record in MongoDB stored as BSON (Binary JSON). It contains key-value pairs like a JSON object. Documents can have nested objects and arrays. Example: a user document might have name, email, address (nested object), and orders (array of objects) - all in one document. Maximum document size is 16MB.
Permalink
Q3:

What is a collection?

Entry

Answer

A collection is a group of documents similar to a table in relational databases but without a fixed schema.
Quick Summary: A collection is a group of documents in MongoDB - roughly equivalent to a table in SQL. But unlike SQL tables, collections have no enforced schema by default - documents in the same collection can have different fields. Collections are created automatically when you insert the first document. You query and index at the collection level.
Permalink
Q4:

What is a database in MongoDB?

Entry

Answer

A database is a container for collections. Each application typically uses one or more databases within the MongoDB server.
Quick Summary: A database in MongoDB is a container for collections. One MongoDB server can host multiple databases. Each database has its own set of files on disk. You switch databases with "use dbname". Common practice: one database per application. Unlike SQL, creating a database just requires inserting data - no explicit CREATE DATABASE needed.
Permalink
Q5:

What is BSON?

Entry

Answer

BSON is a binary format used by MongoDB to store documents. It supports more data types than JSON, such as Date and ObjectId.
Quick Summary: BSON (Binary JSON) is the binary format MongoDB uses to store documents. It extends JSON with additional types: Date, ObjectId, Binary data, 32/64-bit integers, Decimal128. BSON is faster to encode/decode than JSON and supports more data types. When you work with MongoDB drivers, you use JSON-like syntax but the data is stored as BSON internally.
Permalink
Q6:

What is an ObjectId?

Entry

Answer

ObjectId is the default unique identifier for documents. It includes timestamp and machine-specific information to ensure global uniqueness.
Quick Summary: ObjectId is MongoDB's default primary key type - a 12-byte unique identifier automatically generated for the _id field. It encodes: 4-byte timestamp, 5-byte random value (unique per machine/process), 3-byte incrementing counter. This makes ObjectIds roughly sortable by creation time, unique across distributed systems, and generated client-side without DB round-trips.
Permalink
Q7:

What is a schema in MongoDB?

Entry

Answer

MongoDB is schema-flexible, allowing documents with different structures. Schema rules can be enforced using validators when needed.
Quick Summary: MongoDB is schemaless by default - no schema definition required. But you can enforce structure using Schema Validation (JSON Schema rules defined on the collection). This lets you have flexible schemas during development but add validation rules as the app matures. Most applications use Mongoose (Node.js) or similar ODM to define schemas at the application layer.
Permalink
Q8:

What is the purpose of the find() method?

Entry

Answer

The find() method retrieves documents based on filters and supports projection, sorting, and pagination.
Quick Summary: find() queries a collection and returns a cursor of matching documents. Usage: db.users.find({age: {$gt: 18}}). The cursor is lazy - documents are fetched in batches as you iterate. You can chain .sort(), .limit(), .skip(), .project() to shape the results. Without arguments, find() returns all documents in the collection.
Permalink
Q9:

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

Entry

Answer

find() returns multiple documents as a cursor, while findOne() returns only the first matching document.
Quick Summary: find() returns a cursor with all matching documents - you iterate through them. findOne() returns the first matching document directly (not a cursor), or null if none found. Use findOne() when you only need one result and don't want to deal with cursor iteration. It's slightly more efficient when you genuinely only need one document.
Permalink
Q10:

What does the updateOne() function do?

Entry

Answer

updateOne() updates the first matching document using operators like $set, $inc, or $push.
Quick Summary: updateOne() updates the first document matching a filter. Takes two args: filter (which docs to match) and update (what to change). Use $set to change specific fields without replacing the whole document. Returns an object with matchedCount and modifiedCount. If you want to update all matching documents, use updateMany() instead.
Permalink
Q11:

What is a deleteOne() operation?

Entry

Answer

deleteOne() removes the first document matching the filter condition.
Quick Summary: deleteOne() removes the first document matching a filter. db.users.deleteOne({_id: id}) deletes exactly one user. Returns deletedCount. If multiple documents match the filter, only the first found is deleted. For deleting all matching documents, use deleteMany(). Always double-check your filter before running delete operations in production.
Permalink
Q12:

What is field projection in MongoDB?

Entry

Answer

Projection specifies which fields to include or exclude when fetching documents, improving efficiency.
Quick Summary: Field projection controls which fields are returned in query results - reduces data transfer and memory usage. In find(), the second argument is the projection: {name: 1, email: 1} returns only name and email. {password: 0} excludes the password field. You can't mix include and exclude in the same projection (except for _id which can always be excluded).
Permalink
Q13:

What is an index in MongoDB?

Entry

Answer

An index improves search performance on fields. Without indexes, MongoDB performs collection scans.
Quick Summary: An index in MongoDB is a data structure (B-tree) that speeds up queries by allowing MongoDB to find documents without scanning the entire collection. Without an index, every query does a full collection scan (COLLSCAN). Create an index on frequently queried fields: db.users.createIndex({email: 1}). Too many indexes slow down writes.
Permalink
Q14:

What is a primary key in MongoDB?

Entry

Answer

Every document has a unique _id field, which acts as the primary key. MongoDB generates an ObjectId if not provided.
Quick Summary: Every MongoDB document has an _id field that serves as the primary key - it must be unique within the collection. By default, MongoDB auto-generates an ObjectId for _id. You can provide your own _id value (string, int, etc.) but it must be unique. The _id field is always indexed automatically.
Permalink
Q15:

What is a replica set?

Entry

Answer

A replica set is a group of MongoDB servers with redundancy and automatic failover, consisting of one primary and multiple secondaries.
Quick Summary: A replica set is a group of MongoDB servers that maintain the same dataset. One is the primary (handles writes), the rest are secondaries (replicate from primary, can serve reads). If the primary fails, secondaries elect a new primary automatically (failover). Provides high availability and data redundancy. Minimum 3 nodes recommended for proper elections.
Permalink
Q16:

What is sharding in MongoDB?

Entry

Answer

Sharding distributes large datasets across multiple servers for horizontal scaling.
Quick Summary: Sharding distributes data across multiple servers (shards) to handle datasets too large for one machine or write throughput too high for one server. Each shard holds a subset of the data determined by the shard key. A mongos router directs queries to the right shard(s). Config servers store the metadata about which chunks live on which shard.
Permalink
Q17:

What is MongoDB Atlas?

Entry

Answer

MongoDB Atlas is the fully managed cloud service for MongoDB, providing automated scaling, backups, and monitoring.
Quick Summary: MongoDB Atlas is MongoDB's fully managed cloud database service. It runs on AWS, Azure, or GCP. Atlas handles provisioning, backups, monitoring, scaling, security patches, and upgrades automatically. Provides Atlas Search (full-text), Atlas Data Lake, Atlas Charts, and online archive. It's the recommended way to run MongoDB in production - no ops overhead.
Permalink
Q18:

What is the difference between MongoDB and a relational database?

Entry

Answer

MongoDB stores flexible JSON-like documents, while relational databases use structured tables and predefined schemas.
Quick Summary: MongoDB vs relational: MongoDB stores data as flexible documents (no fixed schema), relational uses tables with fixed columns. MongoDB doesn't support joins natively (use $lookup or embed data). MongoDB scales horizontally via sharding; relational typically scales vertically. Relational is better for complex transactions and structured data. MongoDB wins for flexible, hierarchical, and rapidly evolving schemas.
Permalink
Q19:

What is the purpose of the $set operator?

Entry

Answer

$set updates or adds fields without replacing the entire document.
Quick Summary: $set updates specific fields of a document without replacing the whole thing. db.users.updateOne({_id: id}, {$set: {name: "Alice", age: 30}}). Only the specified fields change; other fields stay intact. Without $set, if you pass a plain object MongoDB replaces the entire document (losing all other fields). Always use $set for partial updates.
Permalink
Q20:

What does the $inc operator do?

Entry

Answer

$inc increases or decreases numeric values atomically. Useful for counters or scores.
Quick Summary: $inc atomically increments (or decrements) a numeric field by the given amount. db.products.updateOne({_id: id}, {$inc: {stock: -1, views: 1}}). Decrements if the value is negative. Atomic - safe for concurrent updates (no read-modify-write race condition). Commonly used for counters, inventory tracking, and vote counts.
Permalink

Curated Sets for MongoDB

No curated sets yet. Group questions into collections from the admin panel to feature them here.

People Also Ask - Related MongoDB Questions

What is MongoDB?
What is a document in MongoDB?
What is a collection?
What is a database in MongoDB?
What is BSON?
What is an ObjectId?
Ready to level up? Start Practice