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<!DOCTYPE html>
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<head>
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  <meta name="description"
        content="Chapter 20 of Python Zero to Hero: learn to connect to relational databases with sqlite3, use SQLAlchemy Core & ORM, and manage migrations."/>
  <meta name="keywords"
        content="Python, sqlite3, SQL, SQLAlchemy, ORM, database, CRUD, alembic, schema, sessions"/>
  <meta name="author" content="Luca Bocaletto"/>
  <title>Chapter 20 – Database Access &amp; ORMs | Python Zero to Hero</title>

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    <header class="my-4">
      <h1 class="display-6">Chapter 20: Database Access &amp; ORMs</h1>
      <p class="text-muted">
        Connect to relational databases with built-in sqlite3, explore SQLAlchemy Core &amp; ORM, and manage schema migrations.
      </p>
      <a href="src/chapter20.py" download
         class="btn btn-outline-primary btn-sm btn-py">
        Download <code>chapter20.py</code>
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    </header>

    <!-- Objectives -->
    <section id="objectives" class="mb-5">
      <h2>Objectives</h2>
      <ul>
        <li>Use <code>sqlite3</code> to open connections, execute queries &amp; transactions.</li>
        <li>Perform CRUD operations with raw SQL.</li>
        <li>Define tables &amp; run queries using SQLAlchemy Core.</li>
        <li>Map Python classes to tables via SQLAlchemy ORM.</li>
        <li>Configure and run Alembic migrations.</li>
      </ul>
    </section>

    <!-- 1. sqlite3 Basics -->
    <section id="sqlite3" class="mb-5">
      <h2>1. sqlite3 Basics</h2>
      <pre><code class="python">import sqlite3

# connect (creates file if needed)
conn = sqlite3.connect('app.db')
cur = conn.cursor()

# create table
cur.execute('''
    CREATE TABLE IF NOT EXISTS users (
        id INTEGER PRIMARY KEY,
        name TEXT NOT NULL,
        email TEXT UNIQUE
    )
''')
conn.commit()

# insert and query
cur.execute("INSERT INTO users (name,email) VALUES (?,?)",
            ("Alice","alice@example.com"))
conn.commit()
cur.execute("SELECT id,name FROM users")
print(cur.fetchall())

# cleanup
cur.close()
conn.close()</code></pre>
    </section>

    <!-- 2. Transactions & Context Managers -->
    <section id="transactions" class="mb-5">
      <h2>2. Transactions &amp; Context Managers</h2>
      <pre><code class="python">from sqlite3 import connect

# context manager on connection commits or rolls back
with connect('app.db') as conn:
    cur = conn.cursor()
    cur.execute("UPDATE users SET name=? WHERE id=?",
                ("Alice Smith", 1))

# connection closed automatically</code></pre>
      <p>Exceptions inside the <code>with</code> block trigger rollback.</p>
    </section>

    <!-- 3. SQLAlchemy Core -->
    <section id="core" class="mb-5">
      <h2>3. SQLAlchemy Core</h2>
      <pre><code class="python">from sqlalchemy import (
    create_engine, MetaData, Table, Column, Integer, String, select
)

engine = create_engine('sqlite:///app.db', echo=True)
metadata = MetaData()

users = Table(
    'users', metadata,
    Column('id', Integer, primary_key=True),
    Column('name', String),
    Column('email', String, unique=True)
)
metadata.create_all(engine)

# insert and select
with engine.connect() as conn:
    conn.execute(users.insert(), [
        {"name":"Bob","email":"bob@example.com"}
    ])
    result = conn.execute(select(users)).fetchall()
    print(result)</code></pre>
    </section>

    <!-- 4. SQLAlchemy ORM -->
    <section id="orm" class="mb-5">
      <h2>4. SQLAlchemy ORM</h2>
      <pre><code class="python">from sqlalchemy.orm import (
    declarative_base, sessionmaker
)
from sqlalchemy import Column, Integer, String, create_engine

Base = declarative_base()
engine = create_engine('sqlite:///app.db', echo=False)
Session = sessionmaker(bind=engine)

class User(Base):
    __tablename__ = 'users'
    id    = Column(Integer, primary_key=True)
    name  = Column(String, nullable=False)
    email = Column(String, unique=True)

Base.metadata.create_all(engine)

# CRUD using Session
session = Session()
new_user = User(name="Carol", email="carol@example.com")
session.add(new_user)
session.commit()

users = session.query(User).filter_by(name="Carol").all()
print(users)
session.close()</code></pre>
    </section>

    <!-- 5. Alembic Migrations -->
    <section id="alembic" class="mb-5">
      <h2>5. Alembic Migrations</h2>
      <pre><code class="bash"># install alembic
pip install alembic

# initialize
alembic init migrations

# edit alembic.ini to point to sqlite:///app.db,
# then auto-generate a migration
alembic revision --autogenerate -m "create users table"

# apply migration
alembic upgrade head</code></pre>
      <p>Use migrations to evolve schema safely over time.</p>
    </section>

    <!-- Exercises -->
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      <h2>Exercises</h2>
      <ol>
        <li>Write a script to insert, update &amp; delete records in sqlite3 and print results.</li>
        <li>Recreate the same operations using SQLAlchemy Core.</li>
        <li>Define a <code>Product</code> ORM model and query by price range.</li>
        <li>Create a new Alembic revision to add a &quot;created_at&quot; timestamp column.</li>
      </ol>
    </section>

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