gistfile1.txt

module MMixer
  def m
    puts "Mixing In M"
    self.class.class_eval do
      include M
    end
    m
  end
end

module M
  def m
    puts "m in M"
  end
end

class Foo
  include MMixer
end

>> foo = Foo.new
=> #<Foo:0x1016685f8>

>> foo.m
Mixing In M
m in M
=> nil

>> foo.m
m in M
=> nil