code: function griddb(self, callback) {
  
    if (self.db) {
      return callback(undefined, self.db);
    }
  
    mongo.connect(self.domain, self.user, self.pass, function (err, db) {
  
      self.db = db;
      callback(err, db);
    });
  }
docs: TODO 和 db方法合并