Embedding vs referencing decision in MongoDB - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When to use embedding in MongoDB?

Which scenario is best suited for embedding documents in MongoDB?

A When data needs to be shared across multiple collections

B When related data is very large and updated independently

C When related data is frequently accessed together and has a one-to-few relationship

D When data size exceeds MongoDB document size limit

💡 Hint

Think about data that is tightly connected and often retrieved in one go.

Embedding is ideal when related data is small and accessed together, like an address inside a user document.

🧠 Conceptual

intermediate

When to use referencing in MongoDB?

Which situation calls for referencing documents instead of embedding in MongoDB?

A When related data is large or shared among many documents

B When related data is small and rarely updated

C When data is always accessed together

D When data fits comfortably within a single document

💡 Hint

Consider data that is reused or updated separately.

Referencing is preferred when related data is large or shared across many documents to avoid duplication.

❓ query_result

advanced

Output of embedding vs referencing query

Given two collections, users with embedded addresses and orders referencing users, what is the output of this query?

MongoDB

db.orders.aggregate([
  { $lookup: {
      from: 'users',
      localField: 'user_id',
      foreignField: '_id',
      as: 'user_info'
  }},
  { $unwind: '$user_info' },
  { $project: {
      order_id: 1,
      'user_info.name': 1,
      'user_info.address.city': 1
  }}
])

A []

B [{ order_id: 101, user_info: { name: 'Alice', address: { city: 'Seattle' } } }, { order_id: 102, user_info: { name: 'Bob', address: { city: 'Austin' } } }]

C [{ order_id: 101, user_info: { address: { city: 'Seattle' } } }, { order_id: 102, user_info: { address: { city: 'Austin' } } }]

D [{ order_id: 101, user_info: { name: 'Alice' } }, { order_id: 102, user_info: { name: 'Bob' } }]

💡 Hint

Look at the $project stage selecting name and address.city fields.

The $lookup joins orders with users by user_id. The $project includes order_id, user name, and city from embedded address.

❓ schema

advanced

Choose the best schema design for a blog with comments

You are designing a MongoDB schema for a blog where each post can have hundreds of comments. Which design is best?

A Store comments in a separate collection referencing the post

B Embed only the first 10 comments inside the post, rest in a separate collection

C Embed all comments inside the post document

D Store comments as an array of comment IDs inside the post document

💡 Hint

Consider document size limits and update frequency.

Embedding hundreds of comments can exceed document size limits and cause performance issues. Referencing comments separately is better.

❓ optimization

expert

Optimizing queries with embedding vs referencing

You have a MongoDB collection of products and a collection of reviews referencing products. You want to optimize read performance for showing product details with reviews. Which approach is best?

A Store review IDs inside product and fetch reviews separately in application code

B Keep reviews in a separate collection and use $lookup to join at query time

C Embed all reviews inside each product document

D Embed only the latest 3 reviews inside product, keep others separate

💡 Hint

Balance document size and query speed for common use cases.

Embedding only recent reviews keeps documents manageable and speeds up common queries, while older reviews remain accessible separately.