```ruby
class IndexNamespacesOnVisibilityLevel < ActiveRecord::Migration
  def change
    add_index :namespaces, :visibility_level
  end
end
```

Check if index exists before adding it.
user
```ruby
class IndexNamespacesOnVisibilityLevel < ActiveRecord::Migration
  def change
    unless index_exists?(:namespaces, :visibility_level)
      add_index :namespaces, :visibility_level
    end
  end
end
```
assistant