How to set default database Mongodb in phalcon

Hi all I use mongodb store data, but configuration (mysql ->mongodb) not running.How to

      $config = new Phalcon\Config(array(
          "database" => array(
                  "adapter" => "Mongodb",
                  "host" => "localhost",
                  "username" => "scott",
                  "password" => "cheetah",
                  "dbname" => "test_db"
         ................//
       ));