migration-safety

Migration Safety

CRITICAL: Migrations can destroy production data. This skill prevents catastrophic data loss.

The Problem This Solves

Real disaster scenarios:

• Migration fails halfway, rollback deletes all records
• Re-running migration truncates existing data
• Column drop removes 50,000 user records
• Foreign key constraint fails, cascade deletes everything
• "Fresh migration" on wrong database wipes production

Golden Rules

1. NEVER run migrations without checking for existing data first
2. ALWAYS create a backup before ANY migration
3. NEVER re-run a migration that previously partially completed without investigation
4. ALWAYS verify you're on the correct database/environment
5. NEVER trust migrate:fresh or migrate:reset on any database with data

Pre-Migration Checklist

Step 1: Environment Verification

# Check current environment
echo $NODE_ENV
echo $RAILS_ENV
echo $APP_ENV

# Check database connection
# Laravel
php artisan db:show

# Rails
rails db:version

# Node.js (check connection string)
echo $DATABASE_URL

RED FLAGS - STOP IMMEDIATELY:

• Environment shows "production", "prod", "live"
• Database URL contains production hostname
• You're not 100% certain which database you're connected to

Step 2: Check Existing Data

-- Before ANY migration, check what exists
SELECT table_name,
       (SELECT COUNT(*) FROM information_schema.columns
        WHERE table_name = t.table_name) as columns,
       (SELECT reltuples::bigint FROM pg_class
        WHERE relname = t.table_name) as estimated_rows
FROM information_schema.tables t
WHERE table_schema = 'public'
ORDER BY table_name;

Present this to user:

## Current Database State

**Database:** [name]
**Environment:** [env]
**Tables:** [count]
**Total estimated rows:** [count]

| Table | Columns | Estimated Rows |
|-------|---------|----------------|
| users | 15 | 12,450 |
| orders | 23 | 89,234 |
| ... | ... | ... |

⚠️ This migration will affect a database with EXISTING DATA.