Kernel#qualified_const_get

The question of how to get a class by name comes up with some regularity on the ruby-talk and rails mailing lists. The first response is usually to use Object::const_get. The response to that is that it doesn't handle classes within namespaces, e.g. Foo::Bar. I might argue that Object::const_get("Foo::Bar") should do the right thing and retrieve the value of Bar from the Foo class/module, but the fact of the matter is that it does not. Having an itch to scratch, I wrote Kernel#qualified_const_get:

module Kernel
  def qualified_const_get(str)
    path = str.to_s.split('::')
    from_root = path[0].empty?
    if from_root
      from_root = []
      path = path[1..-1]
    else
      start_ns = ((Class === self)||(Module === self)) ? self : self.class
      from_root = start_ns.to_s.split('::')
    end
    until from_root.empty?
      begin
        return (from_root+path).inject(Object) { |ns,name| ns.const_get(name) }
      rescue NameError
        from_root.delete_at(-1)
      end
    end
    path.inject(Object) { |ns,name| ns.const_get(name) }
  end
end

One of the advantages of this usage is that it handles partially qualified constant names. The following does the right thing in both cases:

require 'qualified_const_get'

module Foo
  module Bar
    class Baz
      def initialize
        puts 'You found me!'
      end
    end
  end
  def self.find_it
    klass = qualified_const_get("Bar::Baz")
    klass.new
  end
  module Quux
    def self.find_it
      klass = qualified_const_get("Bar::Baz")
      klass.new
    end
  end
end

Foo::find_it
Foo::Quux::find_it

Enjoy!