Model definition in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask-SQLAlchemy model query?

Given the model and query below, what will be printed?

Flask

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)

# Assume the database has these users:
# User(id=1, username='alice')
# User(id=2, username='bob')

users = User.query.filter_by(username='alice').all()
print(len(users))

A 1

B 0

C 2

D Raises an AttributeError

💡 Hint

filter_by returns a list of matching records.

The query filters users with username 'alice'. Since there is one such user, the list length is 1.

📝 Syntax

intermediate

Which model definition will cause a syntax error?

Identify the option that will cause a syntax error when defining a Flask-SQLAlchemy model.

A

class Product(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), unique=True)

B

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C

class Product(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), nullable=False)

D

class Product(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), nullable=True)

💡 Hint

Check for missing commas between arguments.

Option C is missing a comma between db.String(100) and nullable=False, causing a syntax error.

❓ state_output

advanced

What is the value of user.email after this code runs?

Consider this model and code snippet. What will be the value of user.email after execution?

Flask

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(120), unique=True, nullable=False)

user = User(email='[e-mail]')
user.email = '[e-mail]'
user.email

A '[e-mail]'

B Raises an AttributeError

C None

D '[e-mail]'

💡 Hint

Assigning a new value to an attribute updates it.

The attribute user.email is updated to '[e-mail]' before accessing it.

🔧 Debug

advanced

Why does this model raise an error when creating the table?

This model raises an error when running db.create_all(). What is the cause?

Flask

class Order(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    total = db.Column(db.Float, nullable=False)
    total = db.Column(db.String(50))

A Missing primary key causes the error.

B Duplicate column name 'total' causes a conflict.

C db.Float is not a valid column type.

D Nullable=False is required for all columns.

💡 Hint

Check if any column names are repeated.

The model defines two columns with the same name 'total', which causes a conflict.

🧠 Conceptual

expert

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

Select the option that correctly sets up 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.Column(db.Integer, db.ForeignKey('book.id'))

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

B

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)

C

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)

D

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'))

💡 Hint

One-to-many uses relationship on the 'one' side and ForeignKey on the 'many' side.

Option D correctly uses db.relationship on Author and db.ForeignKey on Book to link them.

Practice

1. What is the main purpose of defining a model in a Flask application?

easy

A. To represent and manage data structure in the database

B. To create the user interface layout

C. To handle HTTP requests and responses

D. To style the web pages with CSS

Solution

1. Step 1: Understand what a model represents

   A model in Flask defines how data is stored and structured in the database.

2. Step 2: Identify the role of models in the app

   Models help manage data easily by mapping Python classes to database tables.

3. Final Answer:

   To represent and manage data structure in the database -> Option A

4. Quick Check:

   Model = Data structure representation [OK]

Hint: Models map classes to database tables for data storage [OK]

Common Mistakes:

• Confusing models with UI components
• Thinking models handle HTTP requests
• Mixing models with styling or CSS

2. Which of the following is the correct way to define a model class named User with an integer primary key id in Flask-SQLAlchemy?

easy

A. class User():\n id = Column(Integer, primary_key=True)

B. class User(db.Model):\n id = Column(Integer)

C. def User(db.Model):\n id = db.Column(db.Integer, primary_key=True)

D. class User(db.Model):\n id = db.Column(db.Integer, primary_key=True)

Solution

1. Step 1: Check class definition syntax

   Model classes inherit from db.Model in Flask-SQLAlchemy.

2. Step 2: Verify column definition

   Columns use db.Column with type and primary key flag.

3. Final Answer:

   class User(db.Model):\n id = db.Column(db.Integer, primary_key=True) -> Option D

4. Quick Check:

   Model class inherits db.Model and uses db.Column [OK]

Hint: Model classes inherit db.Model; columns use db.Column [OK]

Common Mistakes:

• Forgetting to inherit from db.Model
• Using Column without db prefix
• Defining model as a function instead of class

3. Given this model definition:

class Product(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

p = Product(id=1, name='Book')
print(p.name)

What will be the output?

medium

A. Product object

B. 1

C. Book

D. Error: missing __init__ method

Solution

1. Step 1: Understand model instance creation

   Creating p = Product(id=1, name='Book') sets attributes directly.

2. Step 2: Check what print(p.name) outputs

   It prints the name attribute value, which is 'Book'.

3. Final Answer:

   Book -> Option C

4. Quick Check:

   Accessing attribute prints its value [OK]

Hint: Instance attributes print their assigned values [OK]

Common Mistakes:

• Assuming print shows object type
• Expecting error due to no explicit __init__
• Confusing id and name values

4. What is wrong with this model definition?

class Employee(db.Model):
    id = db.Column(db.Integer)
    name = db.Column(db.String, primary_key=True)

medium

A. db.String requires a length argument

B. Columns must be defined inside a method, not directly

C. Model class must have a __tablename__ attribute

D. Primary key should be on the id column, not name

Solution

1. Step 1: Check db.String usage

   db.String requires a length argument like db.String(100).

2. Step 2: Verify other parts

   Primary key can be on any column; __tablename__ is optional; columns are defined directly.

3. Final Answer:

   db.String requires a length argument -> Option A

4. Quick Check:

   db.String must have length [OK]

Hint: Always specify length in db.String(length) [OK]

Common Mistakes:

• Assuming primary key must be id
• Thinking __tablename__ is mandatory
• Defining columns inside methods

5. You want to define a model Order with a composite primary key consisting of order_id (integer) and product_id (integer). Which is the correct way to define this in Flask-SQLAlchemy?

hard

A. class Order(db.Model):\n order_id = db.Column(db.Integer, primary_key=True)\n product_id = db.Column(db.Integer, primary_key=True, autoincrement=True)

B. class Order(db.Model):\n order_id = db.Column(db.Integer)\n product_id = db.Column(db.Integer)\n __table_args__ = (db.PrimaryKeyConstraint('order_id', 'product_id'),)

C. class Order(db.Model):\n order_id = db.Column(db.Integer, primary_key=True)\n product_id = db.Column(db.Integer)

D. class Order(db.Model):\n order_id = db.Column(db.Integer)\n product_id = db.Column(db.Integer)

Solution

1. Step 1: Understand composite primary keys

   Composite keys require defining multiple columns as primary keys together.

2. Step 2: Check Flask-SQLAlchemy syntax for composite keys

   Use __table_args__ with db.PrimaryKeyConstraint listing columns.

3. Final Answer:

   class Order(db.Model):\n order_id = db.Column(db.Integer)\n product_id = db.Column(db.Integer)\n __table_args__ = (db.PrimaryKeyConstraint('order_id', 'product_id'),) -> Option B

4. Quick Check:

   Composite keys use __table_args__ with PrimaryKeyConstraint [OK]

Hint: Use __table_args__ with PrimaryKeyConstraint for composite keys [OK]

Common Mistakes:

• Setting primary_key=True on multiple columns separately
• Not using __table_args__ for composite keys
• Defining only one column as primary key