Rails - Jun 2006 - validated_associated -- how do I assign invalid collection?

Hi -- if I have a class that declares ''validates_associated''
on a number
of other classes, is it supposed to be relatively easy to replace a 
collection of them?
Not to overwrite them in the DB, but simply to assign them to their 
retrieval method, so that they can be included as part of the owning 
class'' validation.
I mean like:

children_with_potential_errors     Child.update(params[:child].keys,
params[:child].values)
parent.children = children_with_potential_errors
if parent.update_attributes(params[:parent])
  ...
else
  ...
end

I have a class like Parent above, that has three collections 
(one-to-many) like Child above, yet I am having a very diffcult time 
trying to get code like that I have outlined working.
Can anyone think of anything I could be missing that might allow me to 
replace the collection, or maybe someone is aware of a reason that I 
cannot do this?
(If the latter, why would ''validates_asssociated'' exist?)
Any & all help is *very* gratefully received -- I''ve been stuck on
this
for days!
Cheers,
   doug.

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