mongodb连接数

我写了一个网页，很简单。两张静态图片和一段存储在Mongodb中的文字。但是在不断刷新网页时会出现下面的错误： Error: db object already connecting, open cannot be called multiple times at Db.open (X:\BYSJ\Project\simulat\node_modules\mongodb\lib\mongodb\db.js:224:11) at exports.index (X:\BYSJ\Project\simulat\routes\index.js:16:12) at callbacks (X:\BYSJ\Project\simulat\node_modules\express\lib\router\index.js:161:37) at param (X:\BYSJ\Project\simulat\node_modules\express\lib\router\index.js:135:11) at pass (X:\BYSJ\Project\simulat\node_modules\express\lib\router\index.js:142:5) at Router._dispatch (X:\BYSJ\Project\simulat\node_modules\express\lib\router\index.js:170:5) at Object.router (X:\BYSJ\Project\simulat\node_modules\express\lib\router\index.js:33:10) at next (X:\BYSJ\Project\simulat\node_modules\express\node_modules\connect\lib\proto.js:190:15) at Object.methodOverride [as handle] (X:\BYSJ\Project\simulat\node_modules\express\node_modules\connect\lib\middleware\methodOverride.js:37:5) at next (X:\BYSJ\Project\simulat\node_modules\express\node_modules\connect\lib\proto.js:190:15)

我的代码如下： exports.index = function(req, res){ // var dbtest=require('dbcon'); var mongodb=require('mongodb') , mongoserver=new mongodb.Server('localhost',27017) ,dbtest=new mongodb.Db('test',mongoserver,{w:-1});

// console.log(dbtest.serverConfig);
 var pas;
dbtest.open(function(err,db){
    if(err){
       // console.log(db)
        console.log("OPen Db Failed");
        //db.close();
        return res.redirect("/error");
    } else{
      db.collection('test',function(err,coll){
          if(err){
              console.log("Slect Collection  Failed");
              db.close();
              return res.redirect("/error");
          }
        coll.find({title:"test"}).toArray(function(err,doc){
            if(err){
                console.log("query data Failed");
                db.close();
                return   res.redirect("/error")   ;
            }
            pas=doc[0].con;
            //console.log(doc);
            res.locals.pas=pas;
            db.close();
           res.render('index', { title: 'Express' });
        })
      });
    }

});

};

请问问题出在哪啊？