collection_select

<%= collection_select(:category, :id, @categories, :id, :name, options
={:prompt => "-Select categories-", :multiple=>"multiple"},
:class=>"category_dropdown_list", :id=>"collect_select") %>

I'm using a link_to_remote for my submit button. Firstly, by sending
:with => "'categories=' + $('collect_select').serialize()", how will I
access the selected values in my controller? How will I know how many
items are selected and how to retrieve their values? Thanks!