Review

If you're going to do this, you should probably do validates_presence_of :title also. Or change this method to check for nil by doing something like
```self.slug = self.title.parameterize if self.title``` 

Which to do, depends on whether there should always be a title and a slug.

Otherwise if someone saves a question with no title, you'll get ```NoMethodError: undefined method `parameterize' for nil:NilClass```