From: "David A. Black" <dblack@...>
Date: 2009-09-24T20:14:56+09:00
Subject: Re: Monkey Patching 2 Methods, Overrides One Method, Not The Other

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Hi --

On Thu, 24 Sep 2009, MaggotChild wrote:

> On Sep 17, 5:02�am, "David A. Black" <dbl...@rubypal.com> wrote:
>> I think I'd have to see
>> code that exhibits the problem you're describing in order to analyze
>
> This is suited for the Rails group, but the problem can be seen in its
> simplest form here:
>
> #config/initializers/ar_attributes.rb
>
> module ActiveRecord
>  module AttributeMethods
>
>      alias_method :ar_read_attribute, :read_attribute
>      def read_attribute(attr_name)
>        p "read_override"
>        ar_read_attribute(attr_name)
>      end
>
>      alias_method :ar_write_attribute, :write_attribute
>      def write_attribute(attr_name, value)
>        raise 'You made it!'
>      end
>  end
> end
>
> In the Rails console:
>
>>> person.read_attribute :name
> "read_override"
> => "Joe"
>>> person.write_attribute :name, "Bilal"
> => "Bilal"
>>> person.read_attribute :name
> "read_override"
> => "Bilal"

OK, here's what's happening.

On startup, ActiveRecord::Base mixes in a module called Dirty. That
module includes the instruction:

     def self.included(base)
       # ...
       base.alias_method_chain :write_attribute, :dirty
       # ...
     end

At that point, write_attribute is now an instance method of
ActiveRecord::Base -- and your initializer has not yet been run. That
means that when you run the initializer and make the change in the
AttributesMethod module, it's too late: write_attribute has been
copied and put directly in the class.

Of course you can change it in the initializer, if you open AR::Base.
But anyway, that's why the change in the module is not having any
effect.


David

-- 
David A. Black, Director
Ruby Power and Light, LLC (http://www.rubypal.com)
Ruby/Rails training, consulting, mentoring, code review
Book: The Well-Grounded Rubyist (http://www.manning.com/black2)
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