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.

Practice

1. What does a one-to-many relationship in Flask SQLAlchemy represent?

easy

A. One record in a table is linked to many records in another table

B. Many records in a table are linked to one record in another table

C. Two tables have the same number of records

D. One record is linked to exactly one record in another table

Solution

1. Step 1: Understand the meaning of one-to-many

   One-to-many means one item in the first table relates to multiple items in the second table.

2. Step 2: Match the description to options

   One record in a table is linked to many records in another table correctly describes this relationship as one record linked to many records.

3. Final Answer:

   One record in a table is linked to many records in another table -> Option A

4. Quick Check:

   One-to-many = One record linked to many [OK]

Hint: One-to-many means one item connects to many items [OK]

Common Mistakes:

• Confusing one-to-many with many-to-one
• Thinking it means equal number of records
• Assuming one-to-many means one-to-one

2. Which of the following is the correct way to define a one-to-many relationship in Flask SQLAlchemy models?

easy

A. Both B and C together

B. child_id = db.Column(db.Integer, db.ForeignKey('parent.id'))

C. parent = db.relationship('Child', backref='parent', lazy=True)

D. db.relationship('Parent', foreign_key='child.id')

Solution

1. Step 1: Identify relationship and foreign key usage

   In Flask SQLAlchemy, one-to-many needs a relationship on the parent and a foreign key on the child.

2. Step 2: Check options for correct syntax

   parent = db.relationship('Child', backref='parent', lazy=True) shows the relationship on the parent, child_id = db.Column(db.Integer, db.ForeignKey('parent.id')) shows the foreign key on the child. Both are needed together.

3. Final Answer:

   Both B and C together -> Option A

4. Quick Check:

   Relationship + ForeignKey = Correct one-to-many [OK]

Hint: Use relationship on parent and ForeignKey on child [OK]

Common Mistakes:

• Defining only relationship without foreign key
• Using wrong foreign key syntax
• Confusing parent and child roles

3. Given these models, what will author.books return?

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

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

# Assume author is an Author instance with 3 related Book records

medium

A. A single Book object

B. A list of Book objects related to the author

C. An integer count of books

D. null, because books is not defined

Solution

1. Step 1: Understand db.relationship returns

   The relationship attribute returns a list-like collection of related objects in one-to-many.

2. Step 2: Apply to author.books

   Since author.books is a relationship to Book, it returns a list of Book objects linked to that author.

3. Final Answer:

   A list of Book objects related to the author -> Option B

4. Quick Check:

   Relationship attribute returns list of related objects [OK]

Hint: Relationship attribute returns list of related items [OK]

Common Mistakes:

• Expecting a single object instead of a list
• Thinking it returns count instead of objects
• Assuming attribute is undefined

4. Identify the error in this Flask SQLAlchemy one-to-many setup:

class Parent(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    children = db.relationship('Child')

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

medium

A. children should be a db.Column, not relationship

B. Relationship should be on Child, not Parent

C. Missing ForeignKey on Child's parent_id column

D. Parent model missing primary key

Solution

1. Step 1: Check Child model's parent_id column

   It lacks the ForeignKey constraint linking to Parent's id.

2. Step 2: Understand relationship requires ForeignKey

   Without ForeignKey, SQLAlchemy cannot link Child to Parent properly.

3. Final Answer:

   Missing ForeignKey on Child's parent_id column -> Option C

4. Quick Check:

   ForeignKey needed on child foreign key column [OK]

Hint: ForeignKey must be on child foreign key column [OK]

Common Mistakes:

• Forgetting ForeignKey on child side
• Placing relationship on wrong model
• Confusing relationship with column

5. You want to query all books written by an author named 'Alice' using Flask SQLAlchemy with one-to-many models Author and Book. Which code snippet correctly achieves this?

hard

A. author = Author.query.filter_by(name='Alice').all()\nbooks = author.books if author else []

B. books = Book.query.filter(author.name == 'Alice').all()

C. books = Book.query.filter_by(author='Alice').all()

D. books = Book.query.join(Author).filter(Author.name == 'Alice').all()

Solution

1. Step 1: Understand querying with relationships

   To filter books by author name, join Book with Author and filter by Author.name.

2. Step 2: Analyze options for correct query

   books = Book.query.join(Author).filter(Author.name == 'Alice').all() uses join and filter correctly to get all books by 'Alice'. author = Author.query.filter_by(name='Alice').all()\nbooks = author.books if author else [] accesses .books on list of authors (from .all()). books = Book.query.filter(author.name == 'Alice').all() uses invalid reference to 'author'. books = Book.query.filter_by(author='Alice').all() filters by author string incorrectly.

3. Final Answer:

   books = Book.query.join(Author).filter(Author.name == 'Alice').all() -> Option D

4. Quick Check:

   Use join and filter on related model for correct query [OK]

Hint: Use join and filter on related model for queries [OK]

Common Mistakes:

• Using .all() instead of .first() when accessing instance relationship
• Referencing related model without qualifier in filter
• Not joining tables before filtering