Creating a second instance of a singleton class?

ruby-talk
1

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. :slight_smile:

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"

2

you lost me. why wouldn't you just do

   property_versions_class = Class.new(Property){ set_table_name 'property_versions' }

   prop_versions = property_versions_class.new(property.attributes)

??

and that's assuming it had to be dynamic. for that matter why not

   ./script/generate model PropertyVersion

and

   prop_versions = PropertyVersion.new(property.attributes)

??

-a

···

On Thu, 15 Feb 2007, Steve Midgley wrote:

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

--
we can deny everything, except that we have the possibility of being better.
simply reflect on that.
- the dalai lama