Multikey indexes for arrays in MongoDB - Mini Project: Build & Apply

Create and Query Multikey Indexes for Arrays in MongoDB

📖 Scenario: You are managing a MongoDB collection that stores information about books in a library. Each book document contains a title and an array of authors. You want to create an index on the authors array to speed up searches for books by author.

🎯 Goal: Build a MongoDB collection with book documents containing an array of authors. Create a multikey index on the authors field. Then write a query to find all books by a specific author.

📋 What You'll Learn

Create a collection called books with documents having title and authors fields

Insert three book documents with exact titles and authors arrays

Create a multikey index on the authors field

Write a query to find all books where authors array contains 'Jane Austen'

💡 Why This Matters

🌍 Real World

Libraries, bookstores, and content platforms often store multiple authors per book. Multikey indexes help quickly find books by any author.

💼 Career

Database developers and administrators use multikey indexes to optimize queries on array fields in MongoDB, improving application performance.

1

Create the books collection with three book documents

Insert three documents into the books collection with these exact fields and values: { title: 'Pride and Prejudice', authors: ['Jane Austen'] }, { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] }, and { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }.

MongoDB

// Insert the three book documents into the books collection
// Your code here

Hint

Use db.books.insertMany() with an array of three objects, each having title and authors fields.

2

Create a multikey index on the authors field

Create an index on the authors field of the books collection using db.books.createIndex().

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', authors: ['Jane Austen'] },
  { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] },
  { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }
])
// Create a multikey index on authors
// Your code here

Hint

Use db.books.createIndex({ authors: 1 }) to create the multikey index.

3

Write a query to find books by 'Jane Austen'

Write a query using db.books.find() to find all documents where the authors array contains the string 'Jane Austen'.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', authors: ['Jane Austen'] },
  { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] },
  { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }
])
db.books.createIndex({ authors: 1 })
// Find books where authors array contains 'Jane Austen'
// Your code here

Hint

Use db.books.find({ authors: 'Jane Austen' }) to find matching documents.

4

Add a projection to show only the title field in the query result

Modify the previous db.books.find() query to include a projection that returns only the title field and excludes the _id field.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', authors: ['Jane Austen'] },
  { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] },
  { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }
])
db.books.createIndex({ authors: 1 })
// Modify the find query to show only title and exclude _id
// Your code here

Hint

Add a second argument to db.books.find() with { title: 1, _id: 0 } to project only the title.

Practice

1. What is the main purpose of a multikey index in MongoDB when applied to an array field?

easy

A. To index each element inside the array separately for faster queries

B. To store the entire array as a single indexed value

C. To prevent duplicate values inside the array

D. To convert arrays into strings before indexing

Solution

1. Step 1: Understand what a multikey index does

   A multikey index indexes each element of an array field separately, allowing efficient queries on any element inside the array.

2. Step 2: Compare options with this behavior

   Only To index each element inside the array separately for faster queries describes indexing each element separately. Other options describe incorrect behaviors.

3. Final Answer:

   To index each element inside the array separately for faster queries -> Option A

4. Quick Check:

   Multikey index = indexes array elements separately [OK]

Hint: Multikey indexes each array element, not the whole array [OK]

Common Mistakes:

• Thinking multikey indexes the whole array as one value
• Confusing multikey with unique constraints
• Assuming arrays are converted to strings before indexing

2. Which of the following is the correct syntax to create a multikey index on the tags array field in a MongoDB collection named posts?

easy

A. db.posts.createIndex({ tags: [1] })

B. db.posts.createIndex({ tags: 1 })

C. db.posts.createIndex({ tags: 'multikey' })

D. db.posts.createIndex({ tags: { $each: 1 } })

Solution

1. Step 1: Recall MongoDB index creation syntax

   To create an index on a field, use createIndex({ field: 1 }) for ascending order.

2. Step 2: Understand multikey index creation

   MongoDB automatically creates a multikey index if the field is an array. No special keyword is needed.

3. Final Answer:

   db.posts.createIndex({ tags: 1 }) -> Option B

4. Quick Check:

   Index array field normally = multikey index [OK]

Hint: Just index the array field normally; MongoDB handles multikey [OK]

Common Mistakes:

• Using special keywords like 'multikey' in index definition
• Trying to pass array syntax in index keys
• Misusing MongoDB operators inside createIndex

3. Given the collection products with documents like { _id: 1, categories: ["electronics", "gadgets"] }, what will be the result of the query db.products.find({ categories: "gadgets" }) if a multikey index exists on categories?

medium

A. [{ _id: 1, categories: ["electronics"] }]

B. [] (empty array)

C. Error: Cannot query array field

D. [{ _id: 1, categories: ["electronics", "gadgets"] }]

Solution

1. Step 1: Understand query on array field with multikey index

   The query searches for documents where the array contains "gadgets". The multikey index allows fast lookup of such elements.

2. Step 2: Check the document matching the query

   The document with _id 1 has "gadgets" in its categories array, so it matches and will be returned.

3. Final Answer:

   [{ _id: 1, categories: ["electronics", "gadgets"] }] -> Option D

4. Quick Check:

   Query array element with multikey index returns matching docs [OK]

Hint: Query matches docs with array containing the value [OK]

Common Mistakes:

• Expecting empty result because array is indexed
• Thinking query errors on array fields
• Confusing partial array matches with exact matches

4. You created a multikey index on the tags array field, but your query db.articles.find({ tags: "news" }) is slow. What is a likely reason?

medium

A. The index was created on a different field, not tags

B. Multikey indexes do not support queries on array elements

C. You must use $elemMatch to query array fields

D. MongoDB does not index array fields automatically

Solution

1. Step 1: Check index field correctness

   If the index is on a different field, queries on tags won't use the index, causing slowness.

2. Step 2: Verify multikey index behavior

   Multikey indexes do support queries on array elements without special operators like $elemMatch.

3. Final Answer:

   The index was created on a different field, not tags -> Option A

4. Quick Check:

   Index on correct field = fast query [OK]

Hint: Ensure index is on the queried array field [OK]

Common Mistakes:

• Believing multikey indexes don't support element queries
• Thinking $elemMatch is always required
• Assuming MongoDB indexes arrays automatically without createIndex

5. You have a collection users with documents containing an array field roles. You want to create a multikey index on roles but only for users who are active (active: true). Which is the correct way to create a partial multikey index for this?

hard

A. db.users.createIndex({ roles: 1 }, { condition: { active: true } })

B. db.users.createIndex({ roles: 1, active: true })

C. db.users.createIndex({ roles: 1 }, { partialFilterExpression: { active: true } })

D. db.users.createIndex({ roles: 1 }, { filter: { active: true } })

Solution

1. Step 1: Understand partial indexes in MongoDB

   Partial indexes use partialFilterExpression to index only documents matching a filter.

2. Step 2: Apply partialFilterExpression with multikey index

   Creating an index on roles with partialFilterExpression: { active: true } indexes only active users' roles.

3. Final Answer:

   db.users.createIndex({ roles: 1 }, { partialFilterExpression: { active: true } }) -> Option C

4. Quick Check:

   Use partialFilterExpression for conditional multikey index [OK]

Hint: Use partialFilterExpression option for conditional indexes [OK]

Common Mistakes:

• Using invalid options like filter or condition
• Trying to include filter in index key object
• Not knowing partialFilterExpression syntax