Losing sanity, please help!

OK, I’m still trying to move a pic between albums, but this time I’m
going totally ballistic. Please see the debugger session below. The
model is simple, class Pic acts_as_list and belongs_to :album.

Can someone please please please explain to me why @old_alb changes
its value after line 73 ?

######### debugger session:

[64, 73] in ./script/…/config/…/app/controllers/pic_controller.rb
64 redirect_to :action=>:list and return
65 end
66 Debugger.start
67 debugger
=> 69 @[email protected]
70 if @pic.update_attributes(params[:pic])
71 @pic.set_tags @params[:tags]
72 @pic.album =Album.find(params[:album][:id])
73 @pic.save
(rdb:90) n
./script/…/config/…/app/controllers/pic_controller.rb:70 if
(rdb:90) p @old_alb.id
(rdb:90) n
@pic.set_tags @params[:tags]
(rdb:90) n
./script/…/config/…/app/controllers/pic_controller.rb:72 @pic.album
(rdb:90) n
./script/…/config/…/app/controllers/pic_controller.rb:73 @pic.save
(rdb:90) n
./script/…/config/…/app/controllers/pic_controller.rb:74 if
@old_alb != @pic.album
(rdb:90) p @old_alb.id
######### end debugging session

sleepless somewhere.

Variables in Ruby are just references to an Object.
So if you do
a = b
and change b, you change the object b refers to. As a refers tothe
same object, the value of a will change too.

Solution should be (just looked this up, didn’t try and never had
this problem myself before):

a = b.dup

Thanks for your suggestion, but it doesn’t seem true to me, in a
simple case below all works as expected:

foo=Album.find 1
=> #<Album:0xb7186638 @attributes={“created_on”=>“2007-04-25
03:28:30”, “name”=>“top album”, “text”=>nil, “id”=>“1”,
“user_id”=>“1”, “parent_id”=>nil}>

=> #<Album:0xb7186638 @attributes={“created_on”=>“2007-04-25
03:28:30”, “name”=>“top album”, “text”=>nil, “id”=>“1”,
“user_id”=>“1”, “parent_id”=>nil}>

foo=Album.find 2
=> #<Album:0xb717b92c @attributes={“created_on”=>“2007-04-25
03:45:25”, “name”=>“п©я─п╬п╡п╣я─п╨п╟”, “text”=>“п╡п╬я┌ я┌п╟п╨”,
“id”=>“2”, “user_id”=>“1”, “parent_id”=>“1”}>

=> false

So why might it work otherwise in my app?


Your example is different from your real problem:

your problem:

  1. A = Something

  2. B = A

  3. You CHANGE the OBJECT that A refers to.
    A -----
    \ _ Object (If you change B.album, the Object here is
    changed, and as A points to the same Object in memory, it has
    “changed” to
    B -----

  4. As B points to the same object, B has “changed” too (or the object
    it refers to, accuratly spoken)

In your example that you just posted, however, you do something

  1. foo = Something
    2)bar = foo
  2. You set A to point to a NEW, DIFFERENT OBJECT (namely the one
    created by “Album.find 2” )
    foo -------
    \ _ Object (now you don’t change the Object, but you
    create a new one, and make foo point to it)
    bar -------


foo -------> new Object

bar -------> old Object

  1. bar points to the object created in 1) , foo points to the Object
    created in 3)

See the difference in 3) ? in your real problem, you change the OBJECT
that both variables point to.
in your second example, you make “foo” POINT to a different, freshly
created Object.

just give the .dup method a try. and don’t be bothered by my usage of
Caps, i just tried to emphasise the crucial differences :wink:

Thorsten, you’re DA MAN! My sanity is restored, thanks!

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