Question

In Ruby, suppose I have a class Foo to allow me to catalogue my large collection of Foos. It’s a fundamental law of nature that all Foos are green and spherical, so I have defined class methods as follows:

class Foo
  def self.colour
    "green"
  end

  def self.is_spherical?
    true
  end
end

This lets me do

Foo.colour # "green"

but not

my_foo = Foo.new
my_foo.colour # Error!

despite the fact that my_foo is plainly green.

Obviously, I could define an instance method colour which calls self.class.colour, but that gets unwieldy if I have many such fundamental characteristics.

I can also presumably do it by defining method_missing to try the class for any missing methods, but I’m unclear whether this is something I should be doing or an ugly hack, or how to do it safely (especially as I’m actually under ActiveRecord in Rails, which I understand does some Clever Fun Stuff with method_missing).

What would you recommend?

Answer 1

You could define a passthrough facility:

module Passthrough
  def passthrough(*methods)
    methods.each do |method|
      ## make sure the argument is the right type.
      raise ArgumentError if ! method.is_a?(Symbol)
      method_str = method.to_s
      self.class_eval("def #{method_str}(*args) ; self.class.#{method_str}(*args) ; end")
    end
  end
end

class Foo
  extend Passthrough

  def self::colour ; "green" ; end
  def self::is_spherical? ; true ; end
  passthrough :colour, :is_spherical?
end

f = Foo.new
puts(f.colour)
puts(Foo.colour)

I don’t generally like using eval, but it should be pretty safe, here.