连接代码：

function connection(){
   mongoose.connect("mongodb://" + username + ":" + password +"@"+ host + ":" + port + "/" + db);//需要验证账户
   var db = mongoose.connection;
   var options = { user: username, pass: password};
    db.on('error',function callback() {
        console.error.bind(console,'connection error数据库连接失败');
        // console.log("connect close retry connect……");
        db.open(host, db, port, options);
        // console.log("connected");
    });
    
    db.once('open',function callback(){
        console.log('connection success数据库连接成功');
    });   
}

bae的mongodb不支持长连接，不到一分钟就会断开，所以我想当断开的时候再次连接就行。目前是可以重新连接的，但是还是会报错，不知道是不是参数传错了，错误如下：

at EventEmitter.emit (events.js:106:17)
at null.<anonymous> (/home/bae/app/.bae/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:568:14)
at Server.Base._emitAcrossAllDbInstances (/home/bae/app/.bae/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/base.js:504:17)
at emit (/home/bae/app/.bae/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/base.js:254:19)
at Db.EventEmitter.emit (events.js:98:17)
at Db.<anonymous> (/home/bae/app/.bae/node_modules/mongoose/lib/drivers/node-mongodb-native/connection.js:147:10)
at NativeConnection.EventEmitter.emit (events.js:95:17)
at NativeConnection.callback (/home/bae/app/models/connection.js:25:12)
at NativeConnection.Connection.open (/home/bae/app/.bae/node_modules/mongoose/lib/connection.js:212:10)
at NativeConnection.Connection.error (/home/bae/app/.bae/node_modules/mongoose/lib/connection.js:388:24)
TypeError: string is not a function
^
if (callback) return callback(err);
/home/bae/app/.bae/node_modules/mongoose/lib/connection.js:388