From: dblack@...
Date: 2006-12-14T04:50:47+09:00
Subject: Re: How do I catch a missing method on a passed block?

Hi --

On Thu, 14 Dec 2006, Giles Bowkett wrote:

>> Most of what people need to grasp and embrace, in order to understand
>> and use the vast bulk of these techniques, and others, is:
>>
>>    * classes are objects
>>    * each object (or almost each object) has both a "birth class" and a
>>      singleton class
>>    * objects don't have methods; they look for them along a
>>      class/module search path, with well-defined rules of order and
>>      inclusion
>>    * there's one and only one 'self' at every point in the program
>>    * instance variables belong to 'self', always
>>    * class variables are completely unrelated to instance variables,
>>      in spite of the use of @ in one and @@ in the other
>>    * the def and class keywords start new local scopes
>>    * the class keyword can take either a constant (for a named class)
>>      or a "<< object" expression (for a singleton class)
>
> I have to admit, I don't understand what this means in practical
> terms. In fact I have a question I need to find an answer to, and I
> want to post it in this thread, rather than as its own thing, in the
> possibly idealistic hope that somebody can frame an answer in terms of
> these principles, and help me understand them.

If they're not of practical value, then they're not of value.  So I'll
give it a try :-)

> Basically, I need to change a class method. It's the Ferret method
> full_text_search, in a Rails app.
>
> e.g.:
>
> Thing.full_text_search("asdf")
>
> I need to modify this class method. I tried alias, but did it wrong.
>
> alias original_full_text_search full_text_search
>
> but of course it doesn't work, because it's not an instance method but
> a class method. Is this a good place for class_eval? I could be way
> off.
>
> Actually now that I re-read it, I think I do understand the basic
> concepts, I think I'm just weak on the syntax.

In general, if you want to do:

   alias old_x x

you want to do so inside a class definition body for the class where x
is defined (or a subclass of that class).

A "class method" is basically a singleton method of a Class object.
That means that the class where it's defined is the singleton method
of that class object (because that's what singleton classes are: the
places where objects' singleton methods are defined).

In your case, the object in question is Thing.  To get into a class
definition body for a singleton class, you use the class keyword plus
the "<< object" construct:

   class << Thing

You can now perform the aliasing:

     alias original_full_text_search full_text_search
     # redefinitions etc. will follow here

   end

Here's a complete mini-example:

class Thing
   def self.x
     puts "old x"
   end
end

class << Thing
   alias old_x x
   def x
     puts "new x..."
     old_x
   end
end

Thing.x


David

-- 
Q. What's a good holiday present for the serious Rails developer?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    aka The Ruby book for Rails developers!
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)