form including a collection of children


I've searched but with no avail for a solution to my problem. Basically, I'm trying to create a collection select on a parent-child many_to_many relationship (in this case a product has and belongs to many categories)

currently, I've got as far as:

<% form_for :product, product, :url => { :action => 'set_status', :id => product } do |f| %> ...

    <% 0.upto(product.categories.size) do |i| %>       <% fields_for "categories[#{i}]", product.categories[i] do |cat| %>         <%= cat.collection_select( :id, @category_select_collection, :id, :select_option_text, {:prompt => 'Select category'}) %>         <% end %>       <% end %> ... <% end %>

But I get the error: `@categories[0]' is not allowed as an instance variable name. What's the most 'Rails' way to handle this?