One-to-many referencing pattern in MongoDB - Mini Project: Build & Apply

One-to-many referencing pattern in MongoDB

📋 What You'll Learn

💡 Why This Matters

🌍 Real World

Many real-world applications store related data in separate collections or tables and link them using references. For example, a library system stores authors and books separately but connects them.

💼 Career

Understanding one-to-many referencing in MongoDB is essential for database design, data normalization, and efficient querying in backend development and data engineering roles.

1

Create the authors collection with two authors

Create a variable called authors that is a list containing two author documents. Each author document must have an _id field with ObjectId values ObjectId("64b7f1a1f1a1f1a1f1a1f1a1") and ObjectId("64b7f1a1f1a1f1a1f1a1f1a2"), a name field with values "Jane Austen" and "Mark Twain", and a country field with values "UK" and "USA" respectively.

MongoDB

# Create the authors list with two author documents
# Your code here

Hint

Use a list of dictionaries. Use ObjectId for _id values exactly as shown.

2

Create the books collection with three books referencing authors

Create a variable called books that is a list containing three book documents. Each book document must have a title, a year, and an author_id field. Use the exact titles "Pride and Prejudice", "Emma", and "Adventures of Huckleberry Finn". Use years 1813, 1815, and 1884 respectively. Set author_id to reference the correct author's _id from the authors list created in Step 1.

MongoDB

from bson import ObjectId

authors = [
    {"_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1"), "name": "Jane Austen", "country": "UK"},
    {"_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a2"), "name": "Mark Twain", "country": "USA"}
]

# Create the books list with three book documents referencing authors
# Your code here

Hint

Use the ObjectId values from the authors list to set author_id in each book.

3

Insert the authors and books into MongoDB collections

Write two commands to insert the authors list into a collection called authors and the books list into a collection called books. Use the variable db for the database connection. Use insert_many() method for both collections.

MongoDB

from bson import ObjectId

authors = [
    {"_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1"), "name": "Jane Austen", "country": "UK"},
    {"_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a2"), "name": "Mark Twain", "country": "USA"}
]

books = [
    {"title": "Pride and Prejudice", "year": 1813, "author_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1")},
    {"title": "Emma", "year": 1815, "author_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1")},
    {"title": "Adventures of Huckleberry Finn", "year": 1884, "author_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a2")}
]

# Insert authors and books into MongoDB collections
# Your code here

Hint

Use db.authors.insert_many(authors) and db.books.insert_many(books) to insert the lists into collections.

4

Query books with author details using the reference

Write a MongoDB aggregation query using db.books.aggregate() to join the books collection with the authors collection. Use $lookup to match books.author_id with authors._id. Store the result in a variable called books_with_authors. Use $unwind to flatten the joined author array.

MongoDB

from bson import ObjectId

# Assume db is the MongoDB database connection

authors = [
    {"_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1"), "name": "Jane Austen", "country": "UK"},
    {"_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a2"), "name": "Mark Twain", "country": "USA"}
]

books = [
    {"title": "Pride and Prejudice", "year": 1813, "author_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1")},
    {"title": "Emma", "year": 1815, "author_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a1")},
    {"title": "Adventures of Huckleberry Finn", "year": 1884, "author_id": ObjectId("64b7f1a1f1a1f1a1f1a1f1a2")}
]

db.authors.insert_many(authors)
db.books.insert_many(books)

# Write an aggregation query to join books with authors
# Your code here

Hint

Use $lookup with from, localField, foreignField, and as. Then use $unwind on the joined field.

Practice

1. What is the main purpose of the one-to-many referencing pattern in MongoDB?

easy

A. To create a backup of documents automatically

B. To store all related data inside a single document

C. To link one document to many related documents using their IDs

D. To encrypt documents for security

Solution

1. Step 1: Understand one-to-many referencing

   It connects one document to many others by storing their IDs, not embedding all data.

2. Step 2: Compare options

   Only To link one document to many related documents using their IDs describes linking documents by IDs, which is the core of one-to-many referencing.

3. Final Answer:

   To link one document to many related documents using their IDs -> Option C

4. Quick Check:

   One-to-many referencing = linking documents by IDs [OK]

Hint: One-to-many means one document points to many IDs [OK]

Common Mistakes:

• Thinking all data is stored inside one document
• Confusing referencing with embedding
• Assuming it creates backups automatically

2. Which of the following is the correct way to reference many comments in a post document using one-to-many referencing in MongoDB?

easy

A. { _id: 1, title: 'Post', comments: [ObjectId('abc123'), ObjectId('def456')] }

B. { _id: 1, title: 'Post', comments: { text: 'Nice post!' } }

C. { _id: 1, title: 'Post', comments: 'This is a comment' }

D. { _id: 1, title: 'Post', comments: ObjectId('abc123') }

Solution

1. Step 1: Identify referencing format

   One-to-many referencing stores an array of ObjectIds pointing to related documents.

2. Step 2: Check each option

   { _id: 1, title: 'Post', comments: [ObjectId('abc123'), ObjectId('def456')] } uses an array of ObjectIds, which is correct. Others embed data or use a single ID incorrectly.

3. Final Answer:

   { _id: 1, title: 'Post', comments: [ObjectId('abc123'), ObjectId('def456')] } -> Option A

4. Quick Check:

   Array of ObjectIds = correct referencing [OK]

Hint: Use array of ObjectIds to reference many documents [OK]

Common Mistakes:

• Embedding comment text instead of IDs
• Using a single ObjectId for many comments
• Storing comments as plain strings

3. Given these two collections:
posts: [{ _id: 1, title: 'Hello', comments: [101, 102] }]
comments: [{ _id: 101, text: 'Nice!' }, { _id: 102, text: 'Great!' }]
What will be the result of this query to find comments for post with _id 1?
db.comments.find({ _id: { $in: db.posts.findOne({ _id: 1 }).comments } })

medium

A. [] (empty array)

B. [{ _id: 1, title: 'Hello', comments: [101, 102] }]

C. SyntaxError due to nested query

D. [{ _id: 101, text: 'Nice!' }, { _id: 102, text: 'Great!' }]

Solution

1. Step 1: Understand the query

   The inner findOne gets the comments array [101, 102] from post _id 1.

2. Step 2: Use $in to find matching comments

   The outer find returns comments with _id in [101, 102], so both comment documents are returned.

3. Final Answer:

   [{ _id: 101, text: 'Nice!' }, { _id: 102, text: 'Great!' }] -> Option D

4. Quick Check:

   Find comments by IDs in post = matching comment docs [OK]

Hint: Use $in with IDs array to get related documents [OK]

Common Mistakes:

• Expecting post document instead of comments
• Thinking nested query causes syntax error
• Getting empty result by mistake

4. You have this post document:
{ _id: 1, title: 'Test', comments: ['abc', 'def'] }
But when you run db.comments.find({ _id: { $in: post.comments } }), it returns no results. What is the most likely problem?

medium

A. ObjectIds in post.comments are strings, not ObjectId type

B. The comments collection has no documents with those ObjectIds

C. The query syntax is incorrect for $in operator

D. The post document is missing the comments field

Solution

1. Step 1: Check data types of referenced IDs

   If post.comments contains strings instead of ObjectId objects, the query won't match.

2. Step 2: Confirm query correctness

   The query syntax is correct, so the mismatch is likely due to type difference.

3. Final Answer:

   ObjectIds in post.comments are strings, not ObjectId type -> Option A

4. Quick Check:

   Type mismatch in IDs causes no matches [OK]

Hint: Ensure referenced IDs are ObjectId type, not strings [OK]

Common Mistakes:

• Assuming empty results mean syntax error
• Ignoring data type differences
• Thinking missing field causes this error

5. You want to design a blog system with users, posts, and comments. Each user can write many posts, and each post can have many comments. Using one-to-many referencing, which is the best way to organize references?

hard

A. comments store userId; posts store arrays of commentIds

B. posts store userId; comments store postId; users do not store references

C. users embed all posts; posts embed all comments

D. users store arrays of postIds; posts store arrays of commentIds

Solution

1. Step 1: Understand one-to-many referencing best practice

   Usually, the 'many' side stores the reference to the 'one' side to avoid large arrays.

2. Step 2: Apply to blog system

   Posts store userId (one user, many posts), comments store postId (one post, many comments), users don't store arrays.

3. Final Answer:

   posts store userId; comments store postId; users do not store references -> Option B

4. Quick Check:

   Many side stores reference to one side [OK]

Hint: Store reference on 'many' side to 'one' side document [OK]

Common Mistakes:

• Embedding all posts or comments inside users or posts
• Storing large arrays on 'one' side documents
• Confusing which side stores the reference