Ruby/Rails Memoization pattern for dynamic methods created using metaprogramming

memoize.rb

# memoize db/memcache results in instance variable dynamically
def memoize_results(key)
  return instance_variable_get(key) if instance_variable_defined?(key)
  instance_variable_set key, yield  
end

# usage
MY_CONSTANT = [:active, :inactive]
MY_CONSTANT.each { |key|
  define_method("#{key}_users") do
    memoize_results("@#{key}_users") do
      User.send(key).all # assumes that user responds to active, inactive(via scope/filter etc..)
    end
  end
}