# File lib/mongo/mongo_client.rb, line 330
    def db(db_name=nil, opts={})
      if !db_name && uri = ENV['MONGODB_URI']
        db_name = uri[%r{/([^/\?]+)(\?|$)}, 1]
      end

      db_name ||= DEFAULT_DB_NAME

      DB.new(db_name, self, opts)
    end