---
name: database-patterns
description: Database design patterns, migrations with Alembic/Prisma, and query optimization. Use when working with SQL/NoSQL databases or schema migrations.
---

# Database Patterns Skill

## Schema Migrations

### Alembic (Python/SQLAlchemy)

#### Setup
```bash
# Initialize Alembic
alembic init alembic

# Configure alembic.ini
sqlalchemy.url = postgresql://user:pass@localhost/myapp
```

#### alembic/env.py Configuration
```python
from logging.config import fileConfig
from sqlalchemy import engine_from_config, pool
from alembic import context
import os

# Import your models
from app.models import Base

config = context.config

# Override with environment variable
config.set_main_option(
    "sqlalchemy.url",
    os.environ.get("DATABASE_URL", config.get_main_option("sqlalchemy.url"))
)

target_metadata = Base.metadata


def run_migrations_offline() -> None:
    """Run migrations in 'offline' mode."""
    url = config.get_main_option("sqlalchemy.url")
    context.configure(
        url=url,
        target_metadata=target_metadata,
        literal_binds=True,
        dialect_opts={"paramstyle": "named"},
    )

    with context.begin_transaction():
        context.run_migrations()


def run_migrations_online() -> None:
    """Run migrations in 'online' mode."""
    connectable = engine_from_config(
        config.get_section(config.config_ini_section, {}),
        prefix="sqlalchemy.",
        poolclass=pool.NullPool,
    )

    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            target_metadata=target_metadata,
        )

        with context.begin_transaction():
            context.run_migrations()


if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()
```

#### Migration Commands
```bash
# Create migration from model changes
alembic revision --autogenerate -m "add users table"

# Create empty migration
alembic revision -m "add custom index"

# Apply migrations
alembic upgrade head

# Rollback one migration
alembic downgrade -1

# Rollback to specific revision
alembic downgrade abc123

# Show current revision
alembic current

# Show migration history
alembic history --verbose
```

#### Migration Best Practices
```python
# alembic/versions/001_add_users_table.py
"""Add users table

Revision ID: abc123
Revises:
Create Date: 2024-01-15 10:00:00.000000
"""
from typing import Sequence, Union
from alembic import op
import sqlalchemy as sa

revision: str = 'abc123'
down_revision: Union[str, None] = None
branch_labels: Union[str, Sequence[str], None] = None
depends_on: Union[str, Sequence[str], None] = None


def upgrade() -> None:
    op.create_table(
        'users',
        sa.Column('id', sa.UUID(), nullable=False),
        sa.Column('email', sa.String(255), nullable=False),
        sa.Column('name', sa.String(100), nullable=False),
        sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.func.now()),
        sa.Column('updated_at', sa.DateTime(timezone=True), onupdate=sa.func.now()),
        sa.PrimaryKeyConstraint('id'),
    )
    # Create index separately for clarity
    op.create_index('ix_users_email', 'users', ['email'], unique=True)


def downgrade() -> None:
    op.drop_index('ix_users_email', table_name='users')
    op.drop_table('users')
```

#### Data Migrations
```python
"""Backfill user full names

Revision ID: def456
"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.sql import table, column

def upgrade() -> None:
    # Define table structure for data migration
    users = table('users',
        column('id', sa.UUID),
        column('first_name', sa.String),
        column('last_name', sa.String),
        column('full_name', sa.String),
    )

    # Batch update for large tables
    connection = op.get_bind()
    connection.execute(
        users.update().values(
            full_name=users.c.first_name + ' ' + users.c.last_name
        )
    )


def downgrade() -> None:
    # Data migrations typically aren't reversible
    pass
```

### Prisma (TypeScript)

#### Schema Definition
```prisma
// prisma/schema.prisma
datasource db {
  provider = "postgresql"
  url      = env("DATABASE_URL")
}

generator client {
  provider = "prisma-client-js"
}

model User {
  id        String   @id @default(uuid())
  email     String   @unique
  name      String
  role      Role     @default(USER)
  posts     Post[]
  createdAt DateTime @default(now()) @map("created_at")
  updatedAt DateTime @updatedAt @map("updated_at")

  @@map("users")
  @@index([email])
}

model Post {
  id        String   @id @default(uuid())
  title     String
  content   String?
  published Boolean  @default(false)
  author    User     @relation(fields: [authorId], references: [id])
  authorId  String   @map("author_id")
  createdAt DateTime @default(now()) @map("created_at")

  @@map("posts")
  @@index([authorId])
}

enum Role {
  USER
  ADMIN
}
```

#### Migration Commands
```bash
# Create migration from schema changes
npx prisma migrate dev --name add_users_table

# Apply migrations in production
npx prisma migrate deploy

# Reset database (development only)
npx prisma migrate reset

# Generate client
npx prisma generate

# View database
npx prisma studio
```

## SQLAlchemy 2.0 Patterns

### Model Definition
```python
from datetime import datetime
from typing import Optional
from uuid import UUID, uuid4
from sqlalchemy import String, ForeignKey, func
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship


class Base(DeclarativeBase):
    pass


class User(Base):
    __tablename__ = "users"

    id: Mapped[UUID] = mapped_column(primary_key=True, default=uuid4)
    email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
    name: Mapped[str] = mapped_column(String(100))
    role: Mapped[str] = mapped_column(String(20), default="user")
    created_at: Mapped[datetime] = mapped_column(server_default=func.now())
    updated_at: Mapped[Optional[datetime]] = mapped_column(onupdate=func.now())

    # Relationships
    orders: Mapped[list["Order"]] = relationship(back_populates="user")


class Order(Base):
    __tablename__ = "orders"

    id: Mapped[UUID] = mapped_column(primary_key=True, default=uuid4)
    user_id: Mapped[UUID] = mapped_column(ForeignKey("users.id"))
    total: Mapped[int]  # Store as cents
    status: Mapped[str] = mapped_column(String(20), default="pending")
    created_at: Mapped[datetime] = mapped_column(server_default=func.now())

    # Relationships
    user: Mapped["User"] = relationship(back_populates="orders")
    items: Mapped[list["OrderItem"]] = relationship(back_populates="order")
```

### Async Repository Pattern
```python
from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession


class UserRepository:
    def __init__(self, session: AsyncSession):
        self.session = session

    async def get_by_id(self, user_id: UUID) -> User | None:
        result = await self.session.execute(
            select(User).where(User.id == user_id)
        )
        return result.scalar_one_or_none()

    async def get_by_email(self, email: str) -> User | None:
        result = await self.session.execute(
            select(User).where(User.email == email)
        )
        return result.scalar_one_or_none()

    async def list_with_orders(
        self,
        limit: int = 20,
        offset: int = 0
    ) -> list[User]:
        result = await self.session.execute(
            select(User)
            .options(selectinload(User.orders))
            .limit(limit)
            .offset(offset)
        )
        return list(result.scalars().all())

    async def create(self, user: User) -> User:
        self.session.add(user)
        await self.session.flush()
        return user

    async def update(self, user: User) -> User:
        await self.session.flush()
        return user

    async def delete(self, user: User) -> None:
        await self.session.delete(user)
        await self.session.flush()
```

## Query Optimization

### Indexing Strategies
```sql
-- Primary lookup patterns
CREATE INDEX idx_users_email ON users(email);
CREATE INDEX idx_orders_user_id ON orders(user_id);

-- Composite indexes (order matters!)
CREATE INDEX idx_orders_user_status ON orders(user_id, status);

-- Partial indexes
CREATE INDEX idx_orders_pending ON orders(user_id) WHERE status = 'pending';

-- Covering indexes
CREATE INDEX idx_users_email_name ON users(email) INCLUDE (name);
```

### N+1 Query Prevention
```python
# BAD - N+1 queries
users = await session.execute(select(User))
for user in users.scalars():
    print(user.orders)  # Each access triggers a query!

# GOOD - Eager loading
from sqlalchemy.orm import selectinload, joinedload

# Use selectinload for collections
users = await session.execute(
    select(User).options(selectinload(User.orders))
)

# Use joinedload for single relations
orders = await session.execute(
    select(Order).options(joinedload(Order.user))
)
```

### Pagination
```python
from sqlalchemy import select, func


async def paginate_users(
    session: AsyncSession,
    page: int = 1,
    page_size: int = 20,
) -> dict:
    # Count total
    count_query = select(func.count()).select_from(User)
    total = (await session.execute(count_query)).scalar_one()

    # Fetch page
    offset = (page - 1) * page_size
    query = select(User).limit(page_size).offset(offset).order_by(User.created_at.desc())
    result = await session.execute(query)
    users = list(result.scalars().all())

    return {
        "items": users,
        "total": total,
        "page": page,
        "page_size": page_size,
        "has_more": offset + len(users) < total,
    }
```

## NoSQL Patterns (MongoDB)

### Document Design
```python
from pydantic import BaseModel, Field
from datetime import datetime
from bson import ObjectId


class PyObjectId(ObjectId):
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, v):
        if not ObjectId.is_valid(v):
            raise ValueError("Invalid ObjectId")
        return ObjectId(v)


class UserDocument(BaseModel):
    id: PyObjectId = Field(default_factory=PyObjectId, alias="_id")
    email: str
    name: str
    # Embed frequently accessed data
    profile: dict = {}
    # Reference for large/changing data
    order_ids: list[str] = []
    created_at: datetime = Field(default_factory=datetime.utcnow)

    class Config:
        populate_by_name = True
        json_encoders = {ObjectId: str}
```

### MongoDB with Motor (Async)
```python
from motor.motor_asyncio import AsyncIOMotorClient


class MongoUserRepository:
    def __init__(self, client: AsyncIOMotorClient, db_name: str):
        self.collection = client[db_name].users

    async def get_by_id(self, user_id: str) -> dict | None:
        return await self.collection.find_one({"_id": ObjectId(user_id)})

    async def create(self, user: UserDocument) -> str:
        result = await self.collection.insert_one(user.dict(by_alias=True))
        return str(result.inserted_id)

    async def find_by_email_domain(self, domain: str) -> list[dict]:
        cursor = self.collection.find(
            {"email": {"$regex": f"@{domain}$"}},
            {"email": 1, "name": 1}  # Projection
        ).limit(100)
        return await cursor.to_list(length=100)
```

## Migration Safety

### Zero-Downtime Migration Pattern

```python
# Step 1: Add new column (nullable)
def upgrade_step1():
    op.add_column('users', sa.Column('full_name', sa.String(200), nullable=True))

# Step 2: Backfill data (separate deployment)
def upgrade_step2():
    # Run as background job, not in migration
    pass

# Step 3: Make column required (after backfill complete)
def upgrade_step3():
    op.alter_column('users', 'full_name', nullable=False)

# Step 4: Remove old columns (after app updated)
def upgrade_step4():
    op.drop_column('users', 'first_name')
    op.drop_column('users', 'last_name')
```

### Pre-Migration Checklist

- [ ] Backup database before migration
- [ ] Test migration on copy of production data
- [ ] Check migration doesn't lock tables for too long
- [ ] Ensure rollback script works
- [ ] Plan for zero-downtime if needed
- [ ] Coordinate with application deployments

## Commands

```bash
# Alembic
alembic upgrade head           # Apply all migrations
alembic downgrade -1           # Rollback one
alembic history                # Show history
alembic current                # Show current version

# Prisma
npx prisma migrate dev         # Development migration
npx prisma migrate deploy      # Production migration
npx prisma db push             # Push schema without migration

# PostgreSQL
pg_dump -Fc mydb > backup.dump # Backup
pg_restore -d mydb backup.dump # Restore
psql -d mydb -f migration.sql  # Run SQL file
```