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As you well know, mixins only copy plain instance methods into the target class. And may I ask what world-class solutions we have if you need to copy class methods? Um, you can’t copy the metaclass (which includes class methods.)

 >> class GiftedClass
 >>   def self.capabilities
 >>     [:swimming, :diving, :chess, :valour]
 >>   end
 >> end

 >> class LesserClass
 >>   extend (class << GiftedClass; self; end)
 >> end
 TypeError: wrong argument type Class (expected Module)
         from (irb):8:in `extend'
         from (irb):8

I’ve been trying to write a hyperextend method that will take a class and mixin instance and class methods alike, but no.

Nobu’s three-year-old solution is to put all your class methods in a nested module, then alter append_features to allow the mixin to copy the nested module functions into the metaclass. And don’t forget to extend the original module, to make sure the nested module functions are still usable as class (module) methods in the original module.

 module Mix
   def inst_meth
     puts 'inst_meth'
   end

   module ClassMethods
     def class_meth
       puts 'class_meth'
     end
   end

   extend ClassMethods

   def self.append_features(klass)
     super
     klass.extend(ClassMethods)
   end
 end

 class LesserClass
   include Mix
 end

While the behind-the-scenes is a bit wordy, it’s excellent that you can still use include and get the class methods to come through. (Unearthed from ruby-talk:35979.)