From: Steve Midgley <public@...>
Date: 2007-02-15T05:59:15+09:00
Subject: Re: Creating a second instance of a singleton class?

I love this group b/c there are so many clever people here. Ara - 
you're definitely getting me close.

I'll open the bag a little at the expense of brevity so you can see 
what I'm trying to accomplish. I'm using Rails v1.2.2, and in 
particular ActiveRecord 1.15.2.

In the course of a data operation, I have a series of active record 
instances:

instance1ofclass1, instance2ofclass1, instance1ofclass2, 
instance2ofclass2, etc..]

For any given activerecord class for the above list (let's use an 
example table called "property") there is an identical table (less the 
primary key) called "property_versions"

I would like to be able to create a new instance of the activerecord 
model but using a new table name - changing the table_name is handled 
as a class method "set_table_name(new_name)"

So I could say:

#==> assumes we have a "property" instance available..
orig_table = Property.table_name
class Property
   set_table_name 'property_versions'
end

prop_versions = Property.new(property.attributes)

class Property
   set_table_name orig_table
end

But this doesn't seem threadsafe - I could block the whole operation 
but (at the time) it seemed cleaner and simpler to just modify the 
instance of the class rather than the whole parent class.

I may just be an idiot and there's a way easier way to handle this. But 
this was supposed to be a quick little function. Isn't working out like 
that though. :)

Thanks again for any insight,

Steve

ara.t.howard wrote:

> >  harp:~ > cat a.rb
> >  require 'rubygems'
> >  require 'prototype'
> >
> >  # 1)
> >  class Test; end
> >
> >  # 2)
> >  test1 = Test.new
> >
> >  # 3,4)
> >  Test2 = Class.new(test1.class){
> >    def new_method
> >      p 'no singleton class is needed'
> >    end
> >  }
> >
> >  # 5)
> >  test2 = Test2.new
> >
> >  test2.new_method
> >
> >
> >  harp:~ > ruby a.rb
> >  "no singleton class is needed"
>
>and, if it is
>
>    harp:~ > cat a.rb
>    require 'rubygems'
>    require 'prototype'
>
>    # 1)
>    class Test; end
>
>    # 2)
>    test1 = Test.new
>
>    # 3)
>    m = Module.new{
>      def new_method
>        p 'if a singleton class is needed'
>      end
>    }
>    (class << test1; self; end).module_eval{ include m }
>
>    # much easier - no singleton class needed here either
>    # test1.extend m
>
>    # 4)
>    Test2 = Class.new(test1.class){ include m }
>
>    # 5)
>    test2 = Test2.new
>
>    test2.new_method
>
>
>    harp:~ > ruby a.rb
>    "if a singleton class is needed"