One-to-many embedding pattern in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find all authors with their books embedded

Given a MongoDB collection authors where each document embeds an array of books, what is the output of this query?

db.authors.find({}, {name: 1, books: 1, _id: 0})

MongoDB

db.authors.find({}, {name: 1, books: 1, _id: 0})

A [ { _id: ObjectId("...") }, { _id: ObjectId("...") } ]

B [ { name: "Alice" }, { name: "Bob" } ]

C [ { books: [{title: "Book A1"}, {title: "Book A2"}] }, { books: [{title: "Book B1"}] } ]

D [ { name: "Alice", books: [{title: "Book A1"}, {title: "Book A2"}] }, { name: "Bob", books: [{title: "Book B1"}] } ]

💡 Hint

Projection includes name and books, excludes _id.

The query returns all authors with only their name and embedded books fields, excluding the _id field.

🧠 Conceptual

intermediate

Why embed one-to-many data in MongoDB?

Which is the main reason to use embedding for one-to-many relationships in MongoDB?

A To allow multiple collections to share the same document

B To enforce strict relational integrity like foreign keys

C To keep related data together for faster reads and simpler queries

D To reduce document size below 1 KB

💡 Hint

Think about how embedding affects data retrieval speed.

Embedding keeps related data in one document, which makes reading related data faster and queries simpler.

📝 Syntax

advanced

Correct syntax to add a book to an author's embedded books array

Which MongoDB update command correctly adds a new book to the embedded books array of the author named "Alice"?

MongoDB

New book to add: { title: "Book A3", year: 2024 }

A db.authors.updateOne({name: "Alice"}, {$addToSet: {books: {title: "Book A3", year: 2024}}})

B db.authors.updateOne({name: "Alice"}, {$push: {books: {title: "Book A3", year: 2024}}})

C db.authors.updateOne({name: "Alice"}, {$set: {books: {title: "Book A3", year: 2024}}})

D db.authors.updateOne({name: "Alice"}, {$push: {books: [{title: "Book A3", year: 2024}]}})

💡 Hint

Use the operator that adds an element to an array.

The $push operator adds a new element to an array field. Option B uses correct syntax.

🔧 Debug

advanced

Why does this embedded update fail?

Given this update command:

db.authors.updateOne({name: "Alice"}, {$push: {books: {title: "Book A4"}}})

It fails with an error about document size. What is the most likely cause?

A The embedded books array is too large and exceeds MongoDB's 16MB document size limit

B The $push operator requires an array, not an object

C The query filter {name: "Alice"} does not match any document

D The update command is missing the <code>multi: true</code> option

💡 Hint

Think about MongoDB document size limits when embedding many items.

MongoDB documents have a 16MB size limit. Embedding too many books can exceed this limit causing update failure.

❓ optimization

expert

Best approach to model a one-to-many relationship with thousands of related items

You have an author with thousands of books. Embedding all books in one document causes performance and size issues. What is the best MongoDB design pattern to handle this?

A Use referencing: store author in one collection and books in another with author_id field

B Split the books array into multiple embedded arrays inside the author document

C Embed all books anyway and increase MongoDB document size limit

D Store all books as a single large string field inside the author document

💡 Hint

Consider MongoDB document size limits and query performance.

Referencing stores related data in separate collections, avoiding document size limits and improving performance for large one-to-many relationships.