HABTM, select box and array issue

Rails
1

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
product.

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

When I try it I get

undefined method `[]' for :variant_parameter_ids:Symbol
2

Can someone respond please ?