cacher.rb

module Cacher
  def self.prepended target
    target.send :define_singleton_method, :cache do |method|
      Cacher.send :define_method, method do |*args|
        instance_variable_get("@#{method}") || instance_variable_set("@#{method}", super(*args))
      end
    end
  end
end

class Model
  prepend Cacher
  cache :foo

  def foo
    puts 'Model.foo call'
    'result'
  end
end

model = Model.new
puts model.foo
puts model.foo

$> ruby cacher.rb

Model.foo call
result
result