python-database-ops

Python Database Patterns

SQLAlchemy 2.0 and database best practices.

SQLAlchemy 2.0 Basics

from sqlalchemy import create_engine, select
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, Session

class Base(DeclarativeBase):
    pass

class User(Base):
    __tablename__ = "users"

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str] = mapped_column(String(100))
    email: Mapped[str] = mapped_column(String(255), unique=True)
    is_active: Mapped[bool] = mapped_column(default=True)

# Create engine and tables
engine = create_engine("postgresql://user:pass@localhost/db")
Base.metadata.create_all(engine)

# Query with 2.0 style
with Session(engine) as session:
    stmt = select(User).where(User.is_active == True)
    users = session.execute(stmt).scalars().all()

Async SQLAlchemy

from sqlalchemy.ext.asyncio import (
    AsyncSession,
    async_sessionmaker,
    create_async_engine,
)
from sqlalchemy import select

# Async engine
engine = create_async_engine(
    "postgresql+asyncpg://user:pass@localhost/db",
    echo=False,
    pool_size=5,
    max_overflow=10,
)

# Session factory
async_session = async_sessionmaker(engine, expire_on_commit=False)

# Usage
async with async_session() as session:
    result = await session.execute(select(User).where(User.id == 1))
    user = result.scalar_one_or_none()

Model Relationships

from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship, Mapped, mapped_column

class User(Base):
    __tablename__ = "users"

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str]

    # One-to-many
    posts: Mapped[list["Post"]] = relationship(back_populates="author")

class Post(Base):
    __tablename__ = "posts"

    id: Mapped[int] = mapped_column(primary_key=True)
    title: Mapped[str]
    author_id: Mapped[int] = mapped_column(ForeignKey("users.id"))

    # Many-to-one
    author: Mapped["User"] = relationship(back_populates="posts")