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

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

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

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

Hint

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

Practice

1. What does a many-to-many relationship with references mean in MongoDB?

easy

A. Only one collection has references, the other does not.

B. One collection stores all data, the other is empty.

C. Both collections store arrays of IDs pointing to each other.

D. Data is duplicated in both collections without IDs.

Solution

1. Step 1: Understand many-to-many references

   In MongoDB, many-to-many means each document in one collection can link to many documents in another, and vice versa.

2. Step 2: Identify how references are stored

   This is done by storing arrays of IDs in both collections, so each side knows its related documents.

3. Final Answer:

   Both collections store arrays of IDs pointing to each other. -> Option C

4. Quick Check:

   Many-to-many references = arrays of IDs in both collections [OK]

Hint: Many-to-many means both sides hold arrays of IDs [OK]

Common Mistakes:

• Thinking only one side stores references
• Confusing many-to-many with embedding all data
• Assuming data is duplicated without IDs

2. Which of the following is the correct way to store references for a many-to-many relationship between students and courses collections in MongoDB?

easy

A. { _id: 1, name: 'Alice', courses: 101 } and { _id: 101, title: 'Math', students: 1 }

B. { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] }

C. { _id: 1, name: 'Alice', courses: 'Math' } and { _id: 101, title: 'Math', students: 'Alice' }

D. { _id: 1, name: 'Alice', courses: [] } and { _id: 101, title: 'Math', students: [] }

Solution

1. Step 1: Check reference format

   References must be arrays of IDs to represent many-to-many links.

2. Step 2: Validate { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] }

   { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] } shows arrays of IDs in both collections, correctly linking students and courses.

3. Final Answer:

   { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] } -> Option B

4. Quick Check:

   Arrays of IDs in both collections = correct syntax [OK]

Hint: Use arrays of IDs, not single values or names [OK]

Common Mistakes:

• Using single ID instead of array for many-to-many
• Storing names instead of IDs as references
• Leaving reference arrays empty without reason

3. Given these two documents:
{ _id: 1, name: 'Bob', courses: [201, 202] }
{ _id: 201, title: 'History', students: [1, 3] }
What will be the result of querying the courses array for student with _id: 1?

medium

A. ['History']

B. [1, 3]

C. []

D. [201, 202]

Solution

1. Step 1: Identify the student's courses field

   The student document with _id 1 has a courses array: [201, 202].

2. Step 2: Understand the query result

   Querying the courses array for this student returns the IDs [201, 202], not titles or student IDs.

3. Final Answer:

   [201, 202] -> Option D

4. Quick Check:

   Student's courses array = [201, 202] [OK]

Hint: Query returns stored IDs, not linked document fields [OK]

Common Mistakes:

• Confusing course IDs with student IDs
• Expecting course titles instead of IDs
• Assuming empty array if unsure

4. You have these two collections:
students: { _id: 2, name: 'Eve', courses: [301] }
courses: { _id: 301, title: 'Science', students: [] }
What is the problem with this many-to-many reference setup?

medium

A. The students array in courses is missing the student ID.

B. The courses array in students is empty.

C. The student document is missing the courses field.

D. The course document has an invalid _id.

Solution

1. Step 1: Check references in both collections

   The student document lists course 301, but the course document's students array is empty.

2. Step 2: Identify missing reciprocal reference

   For many-to-many, both sides must reference each other. The course's students array should include student ID 2.

3. Final Answer:

   The students array in courses is missing the student ID. -> Option A

4. Quick Check:

   Both sides must reference each other [OK]

Hint: Both sides must list each other's IDs [OK]

Common Mistakes:

• Ignoring missing reciprocal references
• Assuming one side is enough
• Confusing empty arrays with missing fields

5. You want to find all courses taken by students who are enrolled in both course 401 and 402. Given collections with many-to-many references, which MongoDB aggregation pipeline stage helps find students enrolled in both courses?

hard

A. { $match: { courses: { $all: [401, 402] } } }

B. { $match: { courses: { $in: [401, 402] } } }

C. { $match: { courses: 401 } }

D. { $match: { courses: { $size: 2 } } }

Solution

1. Step 1: Understand the query goal

   We want students enrolled in both courses 401 and 402, so their courses array must contain both IDs.

2. Step 2: Choose correct MongoDB operator

   The $all operator matches arrays containing all specified elements, perfect for this case.

3. Final Answer:

   { $match: { courses: { $all: [401, 402] } } } -> Option A

4. Quick Check:

   $all matches arrays containing all elements [OK]

Hint: Use $all to match arrays containing all specified IDs [OK]

Common Mistakes:

• Using $in matches any, not all elements
• Using $size only checks array length, not content
• Matching single course ID misses the second course