From: Gary Weaver <lists@...>
Date: 2012-08-29T22:25:00+09:00
Subject: Re: dynamic class resolution?

Ryan Davis wrote in post #1073629:
> You probably want to look at const_missing. Watch out and make sure you
> always call super because rails uses it to be lazy about requires.

Thanks, Ryan! Looked into that more yesterday and learned about 
ActiveSupport::Dependencies: 
https://github.com/rails/rails/blob/3-2-stable/activesupport/lib/active_support/dependencies.rb

It's funny after using Rails so much that I didn't give a lot of thought 
how class and module resolution work.

So, I thought I'd try to be crafty and throw together a gem to load in 
Rails 3.2 that would do this (among other things):

class Object
  class << self
    class << self
      @ref_classes={}

      puts "renaming const_missing to object_const_missing in #{self}"
      raise "Not on the right class. this is #{self.class.name}" unless 
instance_methods.include?(:const_missing)
      alias_method(:object_const_missing, :const_missing)
      undef_method(:const_missing)
      raise "didn't undefine const_missing on #{self.class.name}" if 
instance_methods.include?(:const_missing)
      raise "didn't change name of const_missing to object_const_missing 
on #{self.class.name}" unless 
instance_methods.include?(:object_const_missing)

      def const_missing(name)
        puts "in my const_missing in #{self}"
        return @ref_classes[name] if @ref_classes[name]
        if name.to_s.end_with?('Ref')
          orig = const_get(name.to_s.chomp('Ref'))
          if orig && orig.ancestors.include?(ActiveRecord::Base)
            ref = orig.dup
            ref.class_eval('include ActiveRecordRefs::Model')
            return ref
          else
            object_const_missing(name)
          end
        end
      end

      raise "should have been able to redefine const_missing on 
#{self.class.name}" unless instance_methods.include?(:const_missing)
    end
  end
end

So, that loads but it is completely ignored, and this morning I read 
this:
http://wondible.com/2011/12/23/give-rails-autoloading-a-boot-to-the-head/

Getting closer. So my new goal would be to have something that allows 
new class "creation" via const_missing that strips off the end and does 
something with it, e.g. if an association has class_name: 
'Person[RO,attr[first_name,last_name]]' then const_missing would take 
that and do a Person.dup and then pass the Person.dup class (named 
'Person[RO,attr[:first_name,:last_name]]') off to a user-configurable 
class's class method that could do something with it like:

# if the RO token is found would use the be_readonly gem's functionality
new_class.class_eval('include BeReadonly::Now')

# parse out attrs and do
new_class.class_eval("self._accessible_attributes[:default] = 
#{attrs.collect{|a|a.to_sym}.inspect}")

It may be too magic, but it would be a way to use associations to have 
each model easily define new classes and functionality to tack on to 
existing models, and would be a lot more DRY. The reason is because 
we're developing another gem that reuses mass assignment security to 
define attributes that are serializable to JSON, along with another 
custom class_attribute that specifies attributes and associations that 
are only serialized.

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