# Migration System Quick Reference

## TL;DR

**Default**: Migrations sind forward-only (nur `up()` Methode).
**Optional**: Implementiere `SafelyReversible` nur wenn Rollback OHNE Datenverlust möglich ist.

## Quick Decision Tree

```
Is rollback safe (no data loss)?
│
├─ YES (e.g., create table, add nullable column, create index)
│  └─ Implement: Migration, SafelyReversible
│
└─ NO (e.g., drop column, transform data, delete data)
   └─ Implement: Migration only
```

## Code Templates

### Forward-Only Migration (Default)

```php
use App\Framework\Database\Migration\Migration;
use App\Framework\Database\Migration\MigrationVersion;
use App\Framework\Database\ConnectionInterface;
use App\Framework\Database\Schema\Schema;

final readonly class YourMigration implements Migration
{
    public function up(ConnectionInterface $connection): void
    {
        $schema = new Schema($connection);
        // Your migration logic
        $schema->execute();
    }

    public function getVersion(): MigrationVersion
    {
        return MigrationVersion::fromTimestamp("2024_12_20_100000");
    }

    public function getDescription(): string
    {
        return 'Description of what this migration does';
    }

    public function getDomain(): string
    {
        return "YourDomain";
    }
}
```

### Safe Rollback Migration

```php
use App\Framework\Database\Migration\{Migration, SafelyReversible};
use App\Framework\Database\Migration\MigrationVersion;
use App\Framework\Database\ConnectionInterface;
use App\Framework\Database\Schema\Schema;

/**
 * This migration is safely reversible because:
 * - [Explain why rollback is safe, e.g., "Creates new empty table"]
 */
final readonly class YourSafeMigration implements Migration, SafelyReversible
{
    public function up(ConnectionInterface $connection): void
    {
        $schema = new Schema($connection);
        // Your migration logic
        $schema->execute();
    }

    public function down(ConnectionInterface $connection): void
    {
        $schema = new Schema($connection);
        // Reverse the changes safely
        $schema->execute();
    }

    public function getVersion(): MigrationVersion
    {
        return MigrationVersion::fromTimestamp("2024_12_20_100000");
    }

    public function getDescription(): string
    {
        return 'Description of what this migration does';
    }

    public function getDomain(): string
    {
        return "YourDomain";
    }
}
```

## Common Operations

### Create Migration

```bash
php console.php make:migration CreateYourTable
```

### Run Migrations

```bash
php console.php db:migrate
```

### Rollback (Only SafelyReversible)

```bash
# Rollback last migration
php console.php db:rollback

# Rollback last 3 migrations
php console.php db:rollback 3
```

### Check Status

```bash
php console.php db:status
```

## Safe vs Unsafe Cheatsheet

| Operation | Safe? | Implements |
|-----------|-------|------------|
| Create table | ✅ | Migration, SafelyReversible |
| Drop table (empty) | ✅ | Migration, SafelyReversible |
| Drop table (with data) | ❌ | Migration only |
| Add nullable column | ✅ | Migration, SafelyReversible |
| Add NOT NULL column (with default) | ⚠️  | Case by case |
| Drop column (empty) | ✅ | Migration, SafelyReversible |
| Drop column (with data) | ❌ | Migration only |
| Rename column | ✅ | Migration, SafelyReversible |
| Change column type | ❌ | Migration only |
| Create index | ✅ | Migration, SafelyReversible |
| Drop index | ✅ | Migration, SafelyReversible |
| Add foreign key | ✅ | Migration, SafelyReversible |
| Drop foreign key | ✅ | Migration, SafelyReversible |
| Transform data | ❌ | Migration only |
| Delete data | ❌ | Migration only |
| Merge tables | ❌ | Migration only |

## Error Handling

### Rollback Failed (Not SafelyReversible)

```bash
❌ Rollback failed: Migration 2024_12_20_100000 does not support safe rollback

💡 Recommendation:
   Create a new forward migration to undo the changes instead:
   php console.php make:migration FixYourChanges
```

### What to do:

1. Create new forward migration
2. Implement reverse logic in `up()` method
3. Run `php console.php db:migrate`

## Testing Rollback

```bash
# Test cycle in development
php console.php db:migrate
php console.php db:rollback 1
php console.php db:migrate  # Should succeed again
```

## Best Practices

1. ✅ **Document why safe**: Always comment why a migration is SafelyReversible
2. ✅ **Test rollback cycle**: Test `up() → down() → up()` in development
3. ✅ **Default to forward-only**: Only add SafelyReversible if certain
4. ❌ **Never rollback in production**: Even "safe" rollbacks should be avoided
5. ✅ **Use fix-forward**: Prefer new migrations over rollback
6. ✅ **Backup before rollback**: Always backup database before rolling back
7. ✅ **Check for data**: Verify column is empty before making it SafelyReversible

## Links

- **Full Examples**: `docs/claude/examples/migrations/SafeVsUnsafeMigrations.md`
- **Database Patterns**: `docs/claude/database-patterns.md`
- **Migration Interface**: `src/Framework/Database/Migration/Migration.php`
- **SafelyReversible Interface**: `src/Framework/Database/Migration/SafelyReversible.php`

## Framework Compliance

✅ Readonly classes: `final readonly class`
✅ Composition over inheritance: Interface-based
✅ Explicit contracts: `SafelyReversible` is opt-in
✅ Type safety: Strong typing throughout
✅ No primitive obsession: Uses Value Objects (MigrationVersion)