Collection management on form updates - problem with updating

I have an AR descendant (job.rb) that contains a collection of other AR
models.

has_many :target_lists, :through => :job_list_associations, :source =>
:target_list

I have a form that I use to update the target_lists array.

Here are the two methods for "target_list_ids" which I use to manage
the form data for job.

  def target_list_ids
    ids = Array.new
    self.target_lists.each do |list|
       ids << list.DataSetID
    end
    ids
  end

  public
  def target_list_ids=(list_array)
    #self.target_lists = Array.new - WHY DOES THIS CAUSE
PROBLEMS?!!?!!?
    self.target_lists.clear
    list_array.each do |id|
      self.target_lists << TargetList.find(id)
    end
  end

On the post, when target_list_ids is called, if I use "clear" on the
target_lists array and then set whatever came in on the form,
everything works fine and I see my changes on the form.

However, if I try to assign self.target_lists to a new Array object and
do the setting, I never see the changes. When the form redisplays
(assuming another error on the page, I see the form without any list
changes I made).

This _almost_ makes sense to me, but I wondered if someone could
explain why setting self.target_lists to be a new Array object seems to
ensure that none of the changes are stored in the job object. Is it
because the "attribute" target_lists is not being updated when I create
the new array? And if I simply manipulate the existing collection (via
clear), I'm directly updating the "attribute" target_lists?

Thanks,
Wes