gistfile1.rb

#===================
# routes:

map.named_filters ':category_slug/*filters'

#===================
# controller

before_filter :load_category, :only => 'show'
before_filter :apply_filters, :only => 'show'

def show
  @products = @category.products
  @applied_filters.each do |descriptor, option|
    # TODO : actual filtering.  named_scope, maybe?
  end

  # other metadata stuff, blah, blah.
end

def load_category
  @category = Category.find_by_slug(params[:category_slug])
end

# grab descriptors / options from filters
def apply_filters
  @applied_filters = {}

  params[:filters].in_groups_of(2) do |(descriptor_slug, option_slug)|
    descriptor = @category.descriptors.find_by_slug(descriptor_slug)
    option = descriptor && descriptor.options.find_by_slug(option_slug)
    @applied_filters[descriptor] = option if option
  end
end

#===================
# helper

# add a descriptor / option pair to the filterset.  if the descriptor is already
# active, replaces the active value with this new one.
def faceted_link(option, descriptors_to_remove=[])
  filters_for_url = @active_filters.dup
  filters_for_url[option.descriptor] = option

  # now canonicalize the URL
  "/#{@category.slug}" + filters_for_url.sort { |a,b|
    # a and b are each a [descriptor, option] pair.  sort by descriptor position
    a.first.position <=> b.first.position
  }.reject {|descriptor, option|
    descriptors_to_remove.include? descriptor
  }.map {|descriptor, option| "/#{descriptor.slug}/#{option.slug}" }.join
end