gistfile1.rb

#http://stackoverflow.com/a/15413611/270511

module Unionable
  extend ActiveSupport::Concern

  module ClassMethods
    def union_scope(*scopes)
      id_column = "#{table_name}.id"

      if (sub_query = scopes.reject { |sc| sc.count == 0 }.map { |s| s.select(id_column).to_sql }.join(" UNION ")).present?
        where "#{id_column} IN (#{sub_query})"
      else
        none
      end
    end
  end
end

Updated to use ActiveSupport::Concern