SQLAlchemy ORM Example Explanation

This document explains the SQLAlchemy ORM example provided in sqlalchemy_example.py.

Overview

The example demonstrates how to:

1. Set up a SQLite database
2. Define a model using SQLAlchemy
3. Create a database session
4. Perform basic CRUD operations (Create and Read)

Code Explanation

Imports and Database Setup

import os
from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

db_file = "sqlalchemy_example.db"
if os.path.exists(db_file):
    os.remove(db_file)

This section imports necessary modules and sets up the SQLite database file. If the file already exists, it's removed to start fresh.

Model Definition

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    username = Column(String(50), unique=True, nullable=False)
    email = Column(String(120), unique=True, nullable=False)

    def __repr__(self):
        return f"<User(username='{self.username}', email='{self.email}')>"

Here, we define the User model. It has three fields: id, username, and email. The repr method provides a string representation of the object.

Database Engine and Session Setup

engine = create_engine(f'sqlite:///{db_file}')
Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()

This code creates the database engine, creates all tables defined in our models, and sets up a session to interact with the database.

CRUD Operations

try:
    new_user = User(username='john_doe', email='john@example.com')
    session.add(new_user)
    session.commit()

    user = session.query(User).filter_by(username='john_doe').first()
    print(f"User: {user.username}, Email: {user.email}")
except Exception as e:
    print(f"An error occurred: {e}")
finally:
    session.close()

This section demonstrates creating a new user (Create operation) and then retrieving that user from the database (Read operation). Error handling is included, and the session is closed in the finally block to ensure proper resource management. Running the Example To run this example:

Ensure you have SQLAlchemy installed:

pip install sqlalchemy

Run the script:

python sqlalchemy_example.py

The script will create a SQLite database file, add a user to it, and then retrieve and print the user's information.