docs: private MongoCredential createCredentail(String group) {
  		final Setting setting = this.setting;
  		if(null == setting) {
  			return null;
  		}
  		final String user = setting.getStr("user", group, setting.getStr("user"));
  		final String pass = setting.getStr("pass", group, setting.getStr("pass"));
  		final String database = setting.getStr("database", group, setting.getStr("database"));
  		return createCredentail(user, database, pass);
  	}