Rails - Dec 2009 - HABTM & collection_ids: confirm changes before saving

It seems that when a HABTM (or has many) is involved, having a
confirmation page prior to saving doesn''t work as seamlessly as other
Rails view/model parings, specifically when confirming an edit. This
is caused by the immediate inserts or deletes that take place upon
assigning to collection_ids.  Changes are saved regardless of whether
the user wants to cancel or proceed.

select_tag ''product[provider_ids][]'',
collection_options_with_selected
#pseudo

def preview
  @product = Product.find params[:id]
  @product.attributes = params[:product]  #provider_ids saved here

  if @product.valid?
    render ''confirm;
  else
    render ''edit''
  end
end

build is not appropriate and push has the same functionality as
assigning to provider_ids.
So I have to do this:

def preview
  @product = Product.find params[:id]
  @product.attributes = params[:product]  #no product

  @selected_providers = Provider.find_by_ids params[:provider_ids]
#to display provider names in confirm page
  #...
end

def update
  @product = Product.find params[:id]
  @product.attributes = params[:product]
  @provider_ids = params[:provider_ids]

  if @product.valid?
    Product.transaction do
      @product.provider_ids = @provider_ids
      @product.save
    end
  else ...
end

There''s no way to do this in the typical Rails attribute assignment
fashion?

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On Dec 31, 11:08 am, MaggotChild
<hsomob1...-/E1597aS9LQAvxtiuMwx3w@public.gmane.org>
wrote:
> def preview
>   @product = Product.find params[:id]
>   @product.attributes = params[:product]  #no product
I meant that no provider_ids are included in params

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