Sequel model plugin that makes faceting easy.

example.txt

ProductType.facet(:name).all
 [{:name=>"Direct Receiver", :count=>9},
  {:name=>"Input System", :count=>6},
  {:name=>"Performance Compressor", :count=>6},
  {:name=>"Air Controller", :count=>5},
  {:name=>"Audible Mount", :count=>5},
  {:name=>"Auto Amplifier", :count=>5},
  {:name=>"Digital Component", :count=>5},
  {:name=>"Direct Transmitter", :count=>5},
  {:name=>"Gel Receiver", :count=>5}...

ProductType.facet { unnest(:tags).as(:name) }.all
 [{:name=>"party", :count=>53},
  {:name=>"seitan", :count=>53},
  {:name=>"squid", :count=>53},
  {:name=>"McSweeney's", :count=>52},
  {:name=>"tattooed", :count=>52},
  {:name=>"art", :count=>50},
  {:name=>"skateboard", :count=>50},
  {:name=>"lo-fi", :count=>49},
  {:name=>"butcher", :count=>48}...

facetable.rb

module Sequel
  module Plugins
    module Facetable
      module DatasetMethods
        def facet(column = nil, &block)
          table = self.model.table_name

          case
          when block_given?
            selector = block
          when column.present?
            selector = proc { column.qualify(table).as(:name) }
          else
            raise ArgumentError.new('You must pass either a column name or a selector block.')
          end

          DB.dataset.with(:facets,
            self.select(&selector).
                 select_append { count(:id.qualify(table)).as(:count) }.
                 group(1)
          ).
          from(:facets).
          order(:count.desc, :name.asc)
        end
      end
    end
  end
end

Remember, that the column you facet by has to either be named, or be aliased to "name".