For these statuses instead of using strings I'd suggest creating constants.  This avoid having "special" values in your code and will give you better errors if some thing is mistyped.

```ruby
class GameSlot
  module Statuses
     Waiting = 'waiting'
     Over = 'over'
     All = constants.map { |status| const_get(status) }
  end
  
  validates :status, inclusion: { in: Statuses::All }
end
```