HABTM, select box and array issue

Product model is as follows
class Product < ActiveRecord::Base
  belongs_to :product_category
  has_and_belongs_to_many :variant_parameters

I am displaying a collection of variant parameters for the product.
Can be more than one selection box. I want to update the same.

I currently have the following selection code
<%= collection_select(:product, :variant_parameter_ids,
@product_categories_variants_parameters, :id, :parameter, {:prompt =>
true}) %>

I have more than one drop down of variant params associated to the

How do I specify that I need :variant_parameter_ids[] and
not :variant_parameter_ids ?

When I try it I get
[code]undefined method `[]' for :variant_parameter_ids:Symbol[/code]

Can someone respond please ?