facet_example.rb

class Project < ActiveRecord::Base
  define_index do
    indexes :name
    indexes :description
    has tags(:id), :as => :tag_ids
  end
end

Project.faceted_search :group_by => :tag_ids
# => { #<Tag id: 1, name: "foo"> => 1, #<Tag id: 2, name: "bar"> => 3 }