Model definition in Flask - Mini Project: Build & Apply

Flask Model Definition

📖 Scenario: You are building a simple web app to store information about books in a library.Each book has a title, an author, and a year it was published.

🎯 Goal: Create a Flask model to represent a Book with the fields title, author, and year_published.

📋 What You'll Learn

Use Flask-SQLAlchemy to define the model

Create a model class named Book

Include fields: id (primary key), title (string), author (string), and year_published (integer)

💡 Why This Matters

🌍 Real World

Defining models is essential for building web apps that store and manage data like books, users, or products.

💼 Career

Understanding model definition with Flask and SQLAlchemy is a key skill for backend web developers working with Python.

1

Set up the Flask app and SQLAlchemy

Create a Flask app instance called app and initialize SQLAlchemy with db = SQLAlchemy(app).

Flask

# Create the Flask app and initialize SQLAlchemy
# Your code here

Hint

Import Flask and SQLAlchemy. Then create app = Flask(__name__) and db = SQLAlchemy(app).

2

Create the Book model class

Define a class called Book that inherits from db.Model.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///library.db'
db = SQLAlchemy(app)

# Define the Book model class
# Your code here

Hint

Define class Book(db.Model): and add columns for id, title, author, and year_published.

3

Add a string representation method

Add a __repr__ method to the Book class that returns a string like <Book 'Title' by Author>.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///library.db'
db = SQLAlchemy(app)

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

    # Add __repr__ method here
    # Your code here

Hint

Define def __repr__(self): and return a formatted string with the book's title and author.

4

Create the database tables

Call db.create_all() to create the tables in the database.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///library.db'
db = SQLAlchemy(app)

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

    def __repr__(self):
        return f""

# Create the database tables
# Your code here

Hint

Use with app.app_context(): then call db.create_all() to create tables.

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