Relationships (one-to-many) in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

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

Given the models Author and Book with a one-to-many relationship, what will be printed by the following code?

Flask

author = Author.query.filter_by(name='Alice').first()
print(len(author.books))

A The number of books written by Alice

B The total number of authors in the database

C The total number of books in the database

D Raises AttributeError because 'books' is not defined

💡 Hint

Remember that 'books' is the related collection for the one-to-many relationship from Author to Book.

The 'books' attribute on the Author model returns a list of Book objects related to that author. So, len(author.books) gives the count of books written by Alice.

📝 Syntax

intermediate

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

Choose the correct way to define a one-to-many relationship between Author and Book models.

A

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    books = db.relationship('Book', backref='author')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))

B

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    author_id = db.Column(db.Integer, db.ForeignKey('book.id'))

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    books = db.relationship('Author', backref='book')

C

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    books = db.relationship('Book', foreign_keys='author.id')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))

D

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    books = db.relationship('Book')

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

💡 Hint

The foreign key should be on the 'many' side, and the relationship on the 'one' side.

Option A correctly places the foreign key on the Book model and defines the relationship on the Author model with a back reference.

🔧 Debug

advanced

Why does accessing author.books raise an error in this code?

Given these models, why does the code 'author = Author.query.first(); print(author.books)' raise an AttributeError?

Flask

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

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))

A Because the backref name is incorrect in the Book model

B Because the foreign key is missing on the Book model

C Because the Author model lacks a relationship() definition for 'books'

D Because the primary key is missing on the Author model

💡 Hint

Check if the Author model knows about its related books.

The Author model does not define a relationship to Book, so 'author.books' does not exist and raises AttributeError.

❓ state_output

advanced

What is the value of book.author.name after this code runs?

Given these models and data, what will 'print(book.author.name)' output?

Flask

author = Author(name='Bob')
db.session.add(author)
db.session.commit()

book = Book(title='Flask Guide', author_id=author.id)
db.session.add(book)
db.session.commit()

book = Book.query.first()
print(book.author.name)

A Flask Guide

B Raises AttributeError

C None

D Bob

💡 Hint

The 'author' attribute on Book is set by the backref from the relationship.

The backref 'author' on Book points to the Author object, so 'book.author.name' returns 'Bob'.

🧠 Conceptual

expert

Which statement best describes the behavior of lazy='dynamic' in a one-to-many relationship?

In Flask-SQLAlchemy, what does setting lazy='dynamic' on a one-to-many relationship do?

A It loads all related items immediately when the parent is queried

B It returns a query object instead of a list, allowing further filtering before loading related items

C It disables loading of related items entirely

D It caches the related items permanently after the first access

💡 Hint

Think about how you can chain filters on the related collection.

Setting lazy='dynamic' returns a query object for the related items, so you can add filters or limits before fetching data.