Question

The REST API railscast has the following code example:

module Api
  module V1
    class ProductsController < ApplicationController
      class Product < ::Product
        def as_json(options={})
          super.merge(released_on: released_at.to_date)
        end
      end
    end
  end
end

I’m having trouble following what:

  class Product < ::Product

…does? When I try to recreate something similar in irb I get:

module Fooirb(main):001:0> module Foobar
irb(main):002:1> class Product < ::Product
irb(main):003:2> end
irb(main):004:1> end
NameError: uninitialized constant Product
    from (irb):2:in `<module:Foobar>'
    from (irb):1

screen casts

Answer 1

The :: token indicates a namespace, and if used without anything directly before, it accesses the top-level namespace. So for example, take this small program:

class Bar
  def initialize
    puts "New ingot created"
  end
end

module Foo
  class Bar
    def initialize(location)
      puts "New bar built in #{location}".
    end
  end

  def self.new_bar(which)
    if which == :top
      Bar.new("Rubytown, USA")
    else
      ::Bar.new
    end
  end
end

If you call Foo.new_bar(:top), the message New bar built in Rubytown, USA gets printed. If instead you use, say, Foo.new_bar(:place_to_drink) it instead prints New ingot created“.