Many-to-many with references in MongoDB - Mini Project: Build & Apply

Many-to-many with references in MongoDB

📖 Scenario: You are building a simple library database. Books can have multiple authors, and authors can write multiple books. You will create collections to store this many-to-many relationship using references.

🎯 Goal: Create two collections authors and books with references to each other to represent many-to-many relationships.

📋 What You'll Learn

Create an authors collection with documents containing _id and name fields.

Create a books collection with documents containing _id, title, and an author_ids array referencing authors._id.

Add a helper variable authorIds to store author ObjectIds for reference.

Insert sample data with two authors and two books, each book referencing one or more authors.

Add a final query to find all books with their author names using $lookup.

💡 Why This Matters

🌍 Real World

Many real-world databases have many-to-many relationships, like books and authors, students and courses, or products and categories.

💼 Career

Understanding how to model and query many-to-many relationships with references is essential for backend developers and database administrators working with MongoDB.

1

Create the authors collection with two authors

Create a variable called authors that is an array with two author documents. Each document should have an _id field with values 1 and 2, and a name field with values 'Alice Walker' and 'James Baldwin' respectively.

MongoDB

# Create the authors array with two documents
# Your code here

Need a hint?

Use a list of dictionaries with exact _id and name values.

2

Create the books collection with references to authors

Create a variable called books that is an array with two book documents. Each document should have an _id, a title, and an author_ids array referencing the author _id values. Use author_ids as the field name for references.

MongoDB

authors = [
    {"_id": 1, "name": "Alice Walker"},
    {"_id": 2, "name": "James Baldwin"}
]
# Create the books array with two documents referencing authors
# Your code here

Need a hint?

Use exact _id and title values and reference author _id in author_ids array.

3

Add a helper variable authorIds for author references

Create a variable called authorIds that is a dictionary mapping author names to their _id values from the authors array. Use the exact keys 'Alice Walker' and 'James Baldwin'.

MongoDB

authors = [
    {"_id": 1, "name": "Alice Walker"},
    {"_id": 2, "name": "James Baldwin"}
]

books = [
    {"_id": 101, "title": "The Color Purple", "author_ids": [1]},
    {"_id": 102, "title": "Go Tell It on the Mountain", "author_ids": [2]}
]

# Create authorIds dictionary mapping author names to _id
# Your code here

Need a hint?

Use a dictionary with exact keys and values matching author _id.

4

Add a MongoDB aggregation query to join books with author names

Create a variable called booksWithAuthors that contains a MongoDB aggregation pipeline using $lookup to join books with authors on author_ids and _id. The pipeline should add a field authors with author documents.

MongoDB

authors = [
    {"_id": 1, "name": "Alice Walker"},
    {"_id": 2, "name": "James Baldwin"}
]

books = [
    {"_id": 101, "title": "The Color Purple", "author_ids": [1]},
    {"_id": 102, "title": "Go Tell It on the Mountain", "author_ids": [2]}
]

authorIds = {
    "Alice Walker": 1,
    "James Baldwin": 2
}

# Create booksWithAuthors aggregation pipeline using $lookup
# Your code here

Need a hint?

Use a list with one dictionary containing the $lookup stage with exact keys and values.