Use database constraints

Don't rely solely on ActiveRecord validations - add database constraints:
- NOT NULL for required fields
- UNIQUE indexes for unique fields
- Foreign key constraints for associations
- Check constraints for business rules

```ruby
# Migration
add_column :users, :email, :string, null: false
add_index :users, :email, unique: true
add_foreign_key :orders, :users
```

Database constraints are your last line of defense against bad data.