# File lib/mongo/util/read_preference.rb, line 19
    def self.mongos(mode, tag_sets)
      if mode != :secondary_preferred || !tag_sets.empty?
        mongos_read_preference = BSON::OrderedHash[:mode => MONGOS_MODES[mode]]
        mongos_read_preference[:tags] = tag_sets if !tag_sets.empty?
      end
      mongos_read_preference
    end