Generally this would be a helper method on the suggestions class or on another helper method, not in the controller.  Also, it's better to use sort_by instead of sort when you are trying to do multi-tier comparisons.  Sort_by would reduce this to something like:

sort_by{ |s| [s.important? ? 0 : 1, s.is_a?(Class) ? s.to_s : s.class.to_s] }