Many-to-many with references in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find all books written by author with _id 'A1'

Given two collections authors and books with many-to-many references, which query returns all books written by the author with _id 'A1'?

MongoDB

db.books.find({ authors: 'A1' })

A db.books.find({ author_id: 'A1' })

B db.authors.find({ books: 'A1' })

C db.books.find({ authors: 'A1' })

D db.books.find({ 'authors._id': 'A1' })

💡 Hint

Books store an array of author IDs in the authors field.

The books collection has an authors array with author IDs. Querying { authors: 'A1' } finds books where 'A1' is in that array.

🧠 Conceptual

intermediate

Why use references for many-to-many relationships in MongoDB?

What is the main reason to use references instead of embedding documents for many-to-many relationships in MongoDB?

A To avoid data duplication and keep data consistent across collections

B Because MongoDB does not support embedding arrays

C To improve query speed by storing all data in one document

D Because references automatically enforce foreign key constraints

💡 Hint

Think about data duplication and consistency when the same data appears in multiple places.

Using references avoids duplicating data in multiple documents, which helps keep data consistent and easier to update.

📝 Syntax

advanced

Correct aggregation to lookup authors for each book

Which aggregation pipeline correctly joins books with authors to get author details for each book?

MongoDB

db.books.aggregate([
  { $lookup: {
      from: 'authors',
      localField: 'authors',
      foreignField: '_id',
      as: 'authorDetails'
  }}
])

A db.books.aggregate([{ $lookup: { from: 'authors', localField: 'authorIds', foreignField: '_id', as: 'authorDetails' }}])

B db.books.aggregate([{ $lookup: { from: 'authors', localField: '_id', foreignField: 'authors', as: 'authorDetails' }}])

C db.books.aggregate([{ $lookup: { from: 'authors', localField: 'authors', foreignField: 'id', as: 'authorDetails' }}])

D db.books.aggregate([{ $lookup: { from: 'authors', localField: 'authors', foreignField: '_id', as: 'authorDetails' }}])

💡 Hint

Match localField with the array field in books and foreignField with the key in authors.

The authors field in books contains author IDs. The _id field in authors matches those IDs. Option D correctly sets these fields.

🔧 Debug

advanced

Why does this query return no results?

Given books documents with authors as an array of ObjectIds, why does this query return no results?

db.books.find({ authors: '507f1f77bcf86cd799439011' })

A Because the <code>authors</code> field is an embedded document, not an array

B Because the query compares string to ObjectId, types must match

C Because the query should use <code>$in</code> operator

D Because the <code>authors</code> field is missing in all documents

💡 Hint

Check the data type of the authors array elements and the query value.

The authors array contains ObjectId values, but the query uses a string. MongoDB requires matching types for equality.

❓ optimization

expert

Best index to optimize many-to-many author-book queries

To optimize queries that find all books by a given author ID stored in the authors array field, which index is best?

A Create a multikey index on the <code>authors</code> field in the <code>books</code> collection

B Create a unique index on the <code>_id</code> field in the <code>authors</code> collection

C Create a compound index on <code>authors</code> and <code>title</code> fields in <code>books</code>

D Create a text index on the <code>authors</code> field in <code>books</code>

💡 Hint

Think about indexing array fields for efficient membership queries.

A multikey index on the authors array allows MongoDB to efficiently find documents where the array contains a specific author ID.