Rails - Aug 2007 - Can't alter (decorate) ActiveRecord::Base#method_missing

Hallo -- I''m trying to decorate the default behaviour of
ActiveRecord::Base#method_missing() with the following code:

<CODE>

class ActiveRecord::Base

  alias_method( :b023_method_missing, :method_missing ) \
    unless method_defined?( :b023_method_missing )

  def method_missing( method, *args )
    puts "\n***** -- In method_missing()"
    b023_method_missing( method, args )
  end

end

</CODE>

This, to my thinking, should simply do what ActiveRecord normally does
with calls to the attributes like "model.name", etc.
However, what is happening is that I am getting a "stack level too
deep"
error.
When run in the console, I get "***** -- In method_missing()" printed
a
million times, then the stack overflow error.
It seems to be that the the call to alias_method is happening twice
(despite the unless guard), thus creating an infinite loop.
Has anyone successfully done something like this with ActiveRecord, and
if so, could they please tell me how!

Cheers,
   Doug.
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I''m not sure why that doesn''t work, unless the method_defined?
call
actually hits the original method_missing.

In any case, is there a reason why you need to use alias_method instead 
of just overwriting the method in your model class? You can then just 
call super() to hit the original method. It''s what inheritance is there
for.
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Bryan Duxbury wrote:
> In any case, is there a reason why you need to use alias_method instead 
> of just overwriting the method in your model class? You can then just 
> call super() to hit the original method. It''s what inheritance is
there
> for.
That''s what I''m looking at now.
Basically, I want to create a plugin that allows delegation of methods 
to encapsulated models upon the raising of NoMethodError in its own 
method_missing() method.
Cheers,
   Doug.
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Then what you probably want to do is define a module that, when 
included, dynamically overwrites the method_missing method. You''d have 
to do an "include MethodMissingWhackiness" in each class that makes
use
of that functionality.

I''m not sure I understand your use case, though. You want to implement 
subsets of your model''s functionality in separate models that will be 
what, nested classes? And you''d like the parent model to transparently 
delegate those methods? That seems somewhat confusing to me.

Maybe instead, you should consider implementing the subsets of methods 
as modules and just including them into the parent class.
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Bryan Duxbury wrote:
> Then what you probably want to do is define a module that, when 
> included, dynamically overwrites the method_missing method. You''d
have
> to do an "include MethodMissingWhackiness" in each class that
makes use
> of that functionality.
> 
> I''m not sure I understand your use case, though. You want to
implement
> subsets of your model''s functionality in separate models that will
be
> what, nested classes? And you''d like the parent model to
transparently
> delegate those methods? That seems somewhat confusing to me.
> 
> Maybe instead, you should consider implementing the subsets of methods 
> as modules and just including them into the parent class.
You could be right.
I think, however, that this could be useful as a simple plugin for those 
situations where you have, say, a Person class that can take on the role 
of Salesman, Customer, Manager, etc.
There is, in fact, a handy plugin that looks like it might answer this 
at:
http://daveverwer.com/2007/3/16/activerecord-delegation

(I''ve just been referred to it by its author on a UK Ruby list.)
> You''d have 
> to do an "include MethodMissingWhackiness" in each class that
makes use
> of that functionality.
Yes -- although to make it look more "Railsy", I''ve reopened 
ActiveRecord::Base and put it in the following method:

def self.delegates_to( delegatee, options )
  # other setup stuff
  include MethodMissingWhackiness
end

& cheers for your input, man!
   Doug.
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Hi again -- just for completeness -- I''ve got what I wanted to happen 
working.
It''s an ongoing thing (so delegated validations are next), but the 
working code so far (after considerable duck-punching) is:

<CODE>
require "active_record"

module Biot023
  module ActsAsDelegator

    def method_missing( method, *args )
      begin
        super
      rescue NoMethodError => err
        self.methods.grep( /^_fetch_delegatee_/ ).each do |mname|
          begin
            model = self.method( mname ).call
            raise( "#{ mname } failed to return model in
method_missing"
) \
              unless model
            _method, _args = [ model.method( method.to_sym ), *args ] \
              rescue _method, _args = [ model.method( :method_missing ), 
[ method, *args ] ]
            return _args ? _method.call( *_args ) : _method.call
          rescue => inner_err
            # do nothing
          end
        end
        raise
      end
    end

  end
end

class ActiveRecord::Base

  # Delegate to a delegatee, whose id must be referenced from this 
model, just
  # the same as a belongs_to() call. In fact, this takes the same 
options as
  # belongs_to(), as it then calls it.
  def self.delegates_to( delegatee, options={} )
    classname = options[:class_name] || delegatee.to_s.camelize
    belongs_to( delegatee.to_sym, options )
    include Biot023::ActsAsDelegator
    class_eval( <<-EOS, "_fetch_delegatee", 1 )
      def _fetch_delegatee_#{ delegatee.to_s }
        self.#{ delegatee.to_s } ||= #{ classname }.new
        self.#{ delegatee.to_s }
      end
    EOS
  end

  before_save do |model|
    model.methods.grep( /^_fetch_delegatee_/ ).each do |mname|
      del = model.method( mname ).call
      del.save!
    end
  end

end

</CODE>

Cheers again,
   Doug.
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