Define secret methods only in parent

method_only_in_parent.rb

module SecretMethods
  def secret_def(name, &block)
    klass = self
    define_method name do
      if self.class != klass
        raise NoMethodError
      else
        yield
      end
    end 
  end
end 

class Parent
  extend SecretMethods
  secret_def(:onlyhere) do
    "bla bla"
  end
end
class Son < Parent
  secret_def(:onlythere) do
    "blu blu"
  end
end
class Grandson < Son
end

puts Parent.new.onlyhere
puts Son.new.onlythere
puts Son.new.onlyhere rescue nil
puts Grandson.new.onlyhere rescue nil
puts Grandson.new.onlythere rescue nil