From: Christopher Dicely <cmdicely@...>
Date: 2009-03-08T17:52:25+09:00
Subject: Re: Can I access or find a object from it's instance variable?

On Sat, Mar 7, 2009 at 6:55 PM, Aki Wakabayashi <zzyzx2001@gmail.com> wrote:
> Hello.
>
> Absolute newb here, and my very first post, so please bear with me...I
> would like to be able to access an object with it's instance variable. I
> simplified my example, but for instance:
>
>
> class Foo
>  attr_accessor :some_id
> end
>
>
> one = Foo.new('7a')
> two = Foo.new('2t')
> three = Foo.new('33')
>
>
> Is it possible to use the instance variable @some_id to access the
> object itself? I was thinking of creating a class method for class Foo
> such as:
>
> class Foo
>  attr_accessor :some_id
>
>  self.find(some_id)
>    # somehow get an array of all the Foo objects, and iterate through
> their
>    # some_id attributes until a match(s) is found, return the object(s)
>  end
> end
>
> ...so I can do something like Foo.find('7a43') to access the object
> 'one'.
>
> I am exhausted from searching how to get an array or hash of objects of
> a given class, and I think I'm going in the wrong direction. Any
> pointers will be greatly appreciated.
>
>
> Thank you in advance :)
> --
> Posted via http://www.ruby-forum.com/.

Others have posted ways using ObjectSpace, which works; its always
seemed to me though that if you know you are going to need to do
something like that with objects of a particular class, that it may
often make more sense to do something like this:

class Foo
  attr_reader :key

  @@instances = Hash.new {|h,k| h[k] = []}

  def self.find(key_val)
    @@instances[key_val].first
  end

  def self.find_all(key_val)
    @@instances[key_val].dup
  end

  def key=(new_key)
    @@instances[@key].delete self
    @@instances[new_key] << self
    @key = new_key
  end

  def initialize(key_val)
    @key = key_val
    @@instances[key_val] << self
  end
end