Initial: Claude config with agents, skills, commands, rules and scripts

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+---
+name: database-migrations
+description: Database migration best practices for schema changes, data migrations, rollbacks, and zero-downtime deployments across PostgreSQL, MySQL, and common ORMs (Prisma, Drizzle, Django, TypeORM, golang-migrate).
+---
+
+# Database Migration Patterns
+
+Safe, reversible database schema changes for production systems.
+
+## When to Activate
+
+- Creating or altering database tables
+- Adding/removing columns or indexes
+- Running data migrations (backfill, transform)
+- Planning zero-downtime schema changes
+- Setting up migration tooling for a new project
+
+## Core Principles
+
+1. **Every change is a migration** — never alter production databases manually
+2. **Migrations are forward-only in production** — rollbacks use new forward migrations
+3. **Schema and data migrations are separate** — never mix DDL and DML in one migration
+4. **Test migrations against production-sized data** — a migration that works on 100 rows may lock on 10M
+5. **Migrations are immutable once deployed** — never edit a migration that has run in production
+
+## Migration Safety Checklist
+
+Before applying any migration:
+
+- [ ] Migration has both UP and DOWN (or is explicitly marked irreversible)
+- [ ] No full table locks on large tables (use concurrent operations)
+- [ ] New columns have defaults or are nullable (never add NOT NULL without default)
+- [ ] Indexes created concurrently (not inline with CREATE TABLE for existing tables)
+- [ ] Data backfill is a separate migration from schema change
+- [ ] Tested against a copy of production data
+- [ ] Rollback plan documented
+
+## PostgreSQL Patterns
+
+### Adding a Column Safely
+
+```sql
+-- GOOD: Nullable column, no lock
+ALTER TABLE users ADD COLUMN avatar_url TEXT;
+
+-- GOOD: Column with default (Postgres 11+ is instant, no rewrite)
+ALTER TABLE users ADD COLUMN is_active BOOLEAN NOT NULL DEFAULT true;
+
+-- BAD: NOT NULL without default on existing table (requires full rewrite)
+ALTER TABLE users ADD COLUMN role TEXT NOT NULL;
+-- This locks the table and rewrites every row
+```
+
+### Adding an Index Without Downtime
+
+```sql
+-- BAD: Blocks writes on large tables
+CREATE INDEX idx_users_email ON users (email);
+
+-- GOOD: Non-blocking, allows concurrent writes
+CREATE INDEX CONCURRENTLY idx_users_email ON users (email);
+
+-- Note: CONCURRENTLY cannot run inside a transaction block
+-- Most migration tools need special handling for this
+```
+
+### Renaming a Column (Zero-Downtime)
+
+Never rename directly in production. Use the expand-contract pattern:
+
+```sql
+-- Step 1: Add new column (migration 001)
+ALTER TABLE users ADD COLUMN display_name TEXT;
+
+-- Step 2: Backfill data (migration 002, data migration)
+UPDATE users SET display_name = username WHERE display_name IS NULL;
+
+-- Step 3: Update application code to read/write both columns
+-- Deploy application changes
+
+-- Step 4: Stop writing to old column, drop it (migration 003)
+ALTER TABLE users DROP COLUMN username;
+```
+
+### Removing a Column Safely
+
+```sql
+-- Step 1: Remove all application references to the column
+-- Step 2: Deploy application without the column reference
+-- Step 3: Drop column in next migration
+ALTER TABLE orders DROP COLUMN legacy_status;
+
+-- For Django: use SeparateDatabaseAndState to remove from model
+-- without generating DROP COLUMN (then drop in next migration)
+```
+
+### Large Data Migrations
+
+```sql
+-- BAD: Updates all rows in one transaction (locks table)
+UPDATE users SET normalized_email = LOWER(email);
+
+-- GOOD: Batch update with progress
+DO $$
+DECLARE
+  batch_size INT := 10000;
+  rows_updated INT;
+BEGIN
+  LOOP
+    UPDATE users
+    SET normalized_email = LOWER(email)
+    WHERE id IN (
+      SELECT id FROM users
+      WHERE normalized_email IS NULL
+      LIMIT batch_size
+      FOR UPDATE SKIP LOCKED
+    );
+    GET DIAGNOSTICS rows_updated = ROW_COUNT;
+    RAISE NOTICE 'Updated % rows', rows_updated;
+    EXIT WHEN rows_updated = 0;
+    COMMIT;
+  END LOOP;
+END $$;
+```
+
+## Prisma (TypeScript/Node.js)
+
+### Workflow
+
+```bash
+# Create migration from schema changes
+npx prisma migrate dev --name add_user_avatar
+
+# Apply pending migrations in production
+npx prisma migrate deploy
+
+# Reset database (dev only)
+npx prisma migrate reset
+
+# Generate client after schema changes
+npx prisma generate
+```
+
+### Schema Example
+
+```prisma
+model User {
+  id        String   @id @default(cuid())
+  email     String   @unique
+  name      String?
+  avatarUrl String?  @map("avatar_url")
+  createdAt DateTime @default(now()) @map("created_at")
+  updatedAt DateTime @updatedAt @map("updated_at")
+  orders    Order[]
+
+  @@map("users")
+  @@index([email])
+}
+```
+
+### Custom SQL Migration
+
+For operations Prisma cannot express (concurrent indexes, data backfills):
+
+```bash
+# Create empty migration, then edit the SQL manually
+npx prisma migrate dev --create-only --name add_email_index
+```
+
+```sql
+-- migrations/20240115_add_email_index/migration.sql
+-- Prisma cannot generate CONCURRENTLY, so we write it manually
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_email ON users (email);
+```
+
+## Drizzle (TypeScript/Node.js)
+
+### Workflow
+
+```bash
+# Generate migration from schema changes
+npx drizzle-kit generate
+
+# Apply migrations
+npx drizzle-kit migrate
+
+# Push schema directly (dev only, no migration file)
+npx drizzle-kit push
+```
+
+### Schema Example
+
+```typescript
+import { pgTable, text, timestamp, uuid, boolean } from "drizzle-orm/pg-core";
+
+export const users = pgTable("users", {
+  id: uuid("id").primaryKey().defaultRandom(),
+  email: text("email").notNull().unique(),
+  name: text("name"),
+  isActive: boolean("is_active").notNull().default(true),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+  updatedAt: timestamp("updated_at").notNull().defaultNow(),
+});
+```
+
+## Django (Python)
+
+### Workflow
+
+```bash
+# Generate migration from model changes
+python manage.py makemigrations
+
+# Apply migrations
+python manage.py migrate
+
+# Show migration status
+python manage.py showmigrations
+
+# Generate empty migration for custom SQL
+python manage.py makemigrations --empty app_name -n description
+```
+
+### Data Migration
+
+```python
+from django.db import migrations
+
+def backfill_display_names(apps, schema_editor):
+    User = apps.get_model("accounts", "User")
+    batch_size = 5000
+    users = User.objects.filter(display_name="")
+    while users.exists():
+        batch = list(users[:batch_size])
+        for user in batch:
+            user.display_name = user.username
+        User.objects.bulk_update(batch, ["display_name"], batch_size=batch_size)
+
+def reverse_backfill(apps, schema_editor):
+    pass  # Data migration, no reverse needed
+
+class Migration(migrations.Migration):
+    dependencies = [("accounts", "0015_add_display_name")]
+
+    operations = [
+        migrations.RunPython(backfill_display_names, reverse_backfill),
+    ]
+```
+
+### SeparateDatabaseAndState
+
+Remove a column from the Django model without dropping it from the database immediately:
+
+```python
+class Migration(migrations.Migration):
+    operations = [
+        migrations.SeparateDatabaseAndState(
+            state_operations=[
+                migrations.RemoveField(model_name="user", name="legacy_field"),
+            ],
+            database_operations=[],  # Don't touch the DB yet
+        ),
+    ]
+```
+
+## golang-migrate (Go)
+
+### Workflow
+
+```bash
+# Create migration pair
+migrate create -ext sql -dir migrations -seq add_user_avatar
+
+# Apply all pending migrations
+migrate -path migrations -database "$DATABASE_URL" up
+
+# Rollback last migration
+migrate -path migrations -database "$DATABASE_URL" down 1
+
+# Force version (fix dirty state)
+migrate -path migrations -database "$DATABASE_URL" force VERSION
+```
+
+### Migration Files
+
+```sql
+-- migrations/000003_add_user_avatar.up.sql
+ALTER TABLE users ADD COLUMN avatar_url TEXT;
+CREATE INDEX CONCURRENTLY idx_users_avatar ON users (avatar_url) WHERE avatar_url IS NOT NULL;
+
+-- migrations/000003_add_user_avatar.down.sql
+DROP INDEX IF EXISTS idx_users_avatar;
+ALTER TABLE users DROP COLUMN IF EXISTS avatar_url;
+```
+
+## Zero-Downtime Migration Strategy
+
+For critical production changes, follow the expand-contract pattern:
+
+```
+Phase 1: EXPAND
+  - Add new column/table (nullable or with default)
+  - Deploy: app writes to BOTH old and new
+  - Backfill existing data
+
+Phase 2: MIGRATE
+  - Deploy: app reads from NEW, writes to BOTH
+  - Verify data consistency
+
+Phase 3: CONTRACT
+  - Deploy: app only uses NEW
+  - Drop old column/table in separate migration
+```
+
+### Timeline Example
+
+```
+Day 1: Migration adds new_status column (nullable)
+Day 1: Deploy app v2 — writes to both status and new_status
+Day 2: Run backfill migration for existing rows
+Day 3: Deploy app v3 — reads from new_status only
+Day 7: Migration drops old status column
+```
+
+## Anti-Patterns
+
+| Anti-Pattern | Why It Fails | Better Approach |
+|-------------|-------------|-----------------|
+| Manual SQL in production | No audit trail, unrepeatable | Always use migration files |
+| Editing deployed migrations | Causes drift between environments | Create new migration instead |
+| NOT NULL without default | Locks table, rewrites all rows | Add nullable, backfill, then add constraint |
+| Inline index on large table | Blocks writes during build | CREATE INDEX CONCURRENTLY |
+| Schema + data in one migration | Hard to rollback, long transactions | Separate migrations |
+| Dropping column before removing code | Application errors on missing column | Remove code first, drop column next deploy |