Delegating instance methods to the class method

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

The Forwardable module that comes with Ruby will do this nicely:

#!/usr/bin/ruby1.8

require 'forwardable'

class Foo

  extend Forwardable

  def self.color
    "green"
  end

  def_delegator self, :color

  def self.is_spherical?
    true
  end

  def_delegator self, :is_spherical?

end

p Foo.color                # "green"
p Foo.is_spherical?        # true
p Foo.new.color            # "green"
p Foo.new.is_spherical?    # true

Answer 2

If it's plain Ruby then using Forwardable is the right answer

In case it's Rails I would have used delegate, e.g.

class Foo
  delegate :colour, to: :class

  def self.colour
    "green"
  end
end

irb(main):012:0> my_foo = Foo.new
=> #<Foo:0x007f9913110d60>
irb(main):013:0> my_foo.colour
=> "green"