Many-to-many relationships in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask SQLAlchemy many-to-many query?

Given two models Author and Book connected by a many-to-many relationship through author_book table, what will print([book.title for book in author.books]) output after adding two books to an author?

Flask

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

author_book = db.Table('author_book',
    db.Column('author_id', db.Integer, db.ForeignKey('author.id')),
    db.Column('book_id', db.Integer, db.ForeignKey('book.id'))
)

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    books = db.relationship('Book', secondary=author_book, back_populates='authors')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100))
    authors = db.relationship('Author', secondary=author_book, back_populates='books')

# Create author and books
author = Author(name='Alice')
book1 = Book(title='Flask Basics')
book2 = Book(title='Advanced Flask')
author.books.append(book1)
author.books.append(book2)

print([book.title for book in author.books])

A ['Alice']

B ['Flask Basics']

C ['Flask Basics', 'Advanced Flask']

D []

💡 Hint

Think about what author.books contains after appending two book objects.

The author.books list contains the two Book objects added. Printing their titles shows both book names.

📝 Syntax

intermediate

Which option correctly defines a many-to-many relationship in Flask SQLAlchemy?

Choose the correct code snippet that defines a many-to-many relationship between Student and Course models using an association table.

A

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    courses = db.relationship('Course', backref='students')

class Course(db.Model):
    id = db.Column(db.Integer, primary_key=True)

B

enrollments = db.Table('enrollments',
    db.Column('student_id', db.Integer, db.ForeignKey('student.id')),
    db.Column('course_id', db.Integer, db.ForeignKey('course.id'))
)

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    courses = db.relationship('Course', secondary=enrollments, back_populates='students')

class Course(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    students = db.relationship('Student', secondary=enrollments, back_populates='courses')

C

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    courses = db.relationship('Course')

class Course(db.Model):
    id = db.Column(db.Integer, primary_key=True)

D

enrollments = db.Table('enrollments',
    db.Column('student_id', db.Integer),
    db.Column('course_id', db.Integer)
)

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    courses = db.relationship('Course', secondary=enrollments)

💡 Hint

Remember the association table needs foreign keys and both sides should use secondary with back_populates.

Option B correctly defines the association table with foreign keys and sets up db.relationship on both models with secondary and back_populates for bidirectional access.

🔧 Debug

advanced

Why does this many-to-many relationship code raise an error?

Consider this code snippet. Why does it raise an error when trying to add a book to an author?

Flask

author_book = db.Table('author_book',
    db.Column('author_id', db.Integer, db.ForeignKey('author.id')),
    db.Column('book_id', db.Integer, db.ForeignKey('book.id'))
)

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    books = db.relationship('Book', secondary=author_book)

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100))

# Usage
author = Author(name='Bob')
book = Book(title='Flask Tips')
author.books.append(book)
db.session.add(author)
db.session.commit()

A Because the Book instance is not added to the session before commit

B Because the association table is missing a primary key

C Because the relationship lacks back_populates causing a runtime error

D Because the author object is missing a name attribute

💡 Hint

Think about what SQLAlchemy needs to track objects before committing.

SQLAlchemy requires all objects to be added to the session before commit. Here, book is not added, so it raises an error.

❓ state_output

advanced

What is the value of len(course.students) after these operations?

Given Student and Course models with a many-to-many relationship, what is the length of course.students after adding two students but removing one?

Flask

enrollments = db.Table('enrollments',
    db.Column('student_id', db.Integer, db.ForeignKey('student.id')),
    db.Column('course_id', db.Integer, db.ForeignKey('course.id'))
)

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    courses = db.relationship('Course', secondary=enrollments, back_populates='students')

class Course(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100))
    students = db.relationship('Student', secondary=enrollments, back_populates='courses')

student1 = Student(name='John')
student2 = Student(name='Jane')
course = Course(title='Math 101')

course.students.append(student1)
course.students.append(student2)
course.students.remove(student1)

print(len(course.students))

A 1

B 2

C 0

D Raises ValueError

💡 Hint

Think about what happens when you remove one student from the list.

After adding two students, removing one leaves exactly one student in the course's students list.

🧠 Conceptual

expert

Which statement best describes the role of the association table in many-to-many relationships?

In Flask SQLAlchemy many-to-many relationships, what is the main purpose of the association table?

A It automatically creates one-to-one relationships between the two models

B It stores all data fields of both related models in one table to optimize queries

C It replaces the need for foreign keys by storing object references directly

D It stores foreign keys linking two tables and allows SQLAlchemy to track their connections without extra model class

💡 Hint

Think about what the association table contains and why it is needed.

The association table holds foreign keys from both related tables to link them, enabling many-to-many relationships without needing a full model class.