From: Robert Klemme <shortcutter@...>
Date: 2009-03-09T06:53:25+09:00
Subject: Re: Can I access or find a object from it's instance variable?

On 08.03.2009 19:01, Christopher Dicely wrote:

>>> What exactly do you mean by "actual identifier"?  If you mean #object_id
>>> then there is method ObjectSpace._id2ref.
>> I meant the variable that I used to point to the object, when they are
>> created like:
>>
>> var = Foo.new(77)
>>
>> I wanted to get access to 'var' by using it's @some_id which is 77.

You can't really as this is a local variable and you would have to know 
the scope (or binding) where to search - the name alone is not 
sufficient.  However, for the problem you are trying to solve this is 
not necessary.

>>> The question is: what are you trying to achieve?
>> Thank you for asking. I was wondering how to update relations(imagine a
>> social networking site) between objects of a single Class. I hope this
>> will clarify why I want to access the variable of the object with its
>> instance variable. This how far I got last night:
>>
>> class Foo
>>
>>  attr_accessor :name, :some_id, :relations
>>
>>  def self.update_relationship(foo1, foo2)
>>    # code that adds @relations for foo1, and foo2 and updates it with
>>    # it's own MINUS its own :some_id, and updates other @some_id's
>> included
>>    # in foo1.relations + foo2.relations.
>>  end
>>
>>  def self.find_by_some_id(some_id)
>>    found = nil
>>    ObjectSpace.each_object(Foo) { |o| found = o if o.oid == oid}
>>    found
>>  end
>>
>>  def to_s
>>    @name
>>  end
>>
>> end
>>
>>
>> (create two objects):
>>
>> John = Foo.new('John', 1,[])
>> Dave = Foo.new('Dave', 2,[])
>>
>> (create a relationship between the two):
>>
>> Foo.update_relationship(John, Dave)
>>
>> John.relations -> [2]
>> Dave.relations -> [1]
>>
>>
>> Let's assume that along the line more objects are created and a new
>> relationship is created with John.(I'm just going to create a new object
>> and populate it's @relations instance variable, so the variables are
>> visible)
>>
>> Steve = Foo.new('Steve', 104, [55, 77])
>>
>> Foo.update_relationship(Steve, John)
>>
>> John.relations -> [2, 55, 77, 104]
>> Steve.relations -> [1,2,55,77]
>>
>> ...well, that's good, but all the other objects (with @some_id 2, 55 and
>> 77) would have to be updated right? For starters, Dave, the second
>> object (with @some_id = 2) is currently still:
>>
>> Dave.relations -> [1]
>>
>> Where it has to be updated to [1, 55, 77, 104]
>>
>> So in order to access the Dave object, I only had 2, it's @some_id
>> instance variable to access it. Now, since I included the variable name
>> in the instance variable I can access the Dave object with the
>> find_by_some_id class method, and
>> get the @name instance variable which is the same as the variable (THIS
>> IS WHERE I THOUGHT IT WAS NOT A RUBYIST SOLUTION)
>>
>> I'm going to read a little more and see if I can use the object's
>> #object_id to do all of this.

> Note that because Ruby objects are passed as references, you generally
> don't need to keep relationships using an identifier, you can just
> keep a list of the "related" objects in an instance variable (the code
> becomes less clear when you store an identifier instead of the object
> itself, you probably aren't saving space, and you are probably hurting
> performance by necessitating a method call to lookup the actual object
> from the identifier.)

Adding to that, Aki, if you need to maintain relationships more often 
then you could follow a similar approach as has been taken with 
attr_accessor and write methods that generate the code for relationship 
maintenance.  E.g.

class Module
   def one_to_one(name, cl)
     module_eval %Q{
       def #{name}=(x)
         # logic to set relationship consistently
       end

       def #{name}
         @#{name}
       end
     }

     cl.module_eval %Q{
        # similar for the other side
     }
   end
end

and then

class Foo
   one_to_one :parent, Foo
end

I guess something like this does exist already.  You can check the RAA.

If you use ActiveRecord for persistence you get these things for free.

Kind regards

	robert