Rails core - Oct 2012 - ActiveRecord: updating an association of an already persisted instance in memory without making the update persist right away. Is this possible?

Hi,

I have ClassA, which has many objects of ClassB. Let''s say that the 
dependency of that relationship is of the destroy kind:

class ClassA < ActiveRecord::Base
    has_many :class_bs, :dependency => :destroy
end
class ClassB < ActiveRecord::Base
    belongs_to :class_a
end

Now let''s say I already have a persisted instance of ClassA, and this 
instance has also associated persisted objects:

   a = ClassA.first
   a.persisted #=> true
   a.class_bs.length #=> >1
   a.class_bs,all?{|b|b.persisted?} #=> true

So now, I would like to replace my associated class_bs with a new 
collection of only one element, which is not persisted. I would like to 
keep this only in memory, until I save the parent (or its child, for all I 
care):

   new_bs = [ClassB.new]
   new_bs.all?{|b| b.new_record? } #=> true
   a.class_bs = new_bs
   a.class_bs.all?{|b| b.new_record?} #=> true
   a.save
   a.class_bs.all?{|b| b.new_record?} #=> false

Problem is, the way it works, step 4 will return "false" instead of
true.
That is, as soon as I assign something to an already persisted parent, the 
new associations are stored right away (unless they are invalid). This is 
something that I don''t like, because I may want to associate a to
another
object x, and I want to validate everything before I persist the changes, 
but the way it works, as soon as I get to x, changes were already made. Is 
there a way to do this in AR? and still keep the dependency => destroy? 

Thanks in advance

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