nodejs使用connect-mongodb报错（Please ensure that you set the default write concern）

connect-mongo

connect-mongodb

G:\nodejs\moviesite>grunt
Running "concurrent:tasks" (concurrent) task
Running "nodemon:dev" (nodemon) task
Running "watch" task
Waiting...
[nodemon] v1.3.7
[nodemon] to restart at any time, enter `rs`
[nodemon] watching: *.*
[nodemon] starting `node app.js`
============================================================================
============
=  Please ensure that you set the default write concern for the database by
setting    =
=   one of the options
=
=
=
=     w: (value of > -1 or the string 'majority'), where < 1 means
=
=        no write acknowledgement
=
=     journal: true/false, wait for flush to journal before acknowledgement
=
=     fsync: true/false, wait for flush to file system before acknowledgemen
t           =
=
=
=  For backward compatibility safe is still supported and
=
=   allows values of [true | false | {j:true} | {w:n, wtimeout:n} | {fsync:t
rue}]      =
=   the default value is false which means the driver receives does not
=
=   return the information of the success/error of the insert/update/remove
=
=
=
=   ex: new Db(new Server('localhost', 27017), {safe:false})
=
=
=
=   http://www.mongodb.org/display/DOCS/getLastError+Command =
=
=
=  The default of no acknowledgement will change in the very near future
=
=
=
=  This message will disappear when the default safe is set on the driver Db
=
============================================================================
============
Sat, 27 Jun 2015 12:49:12 GMT express-session deprecated undefined resave option
; provide resave option at app.js:20:9
Sat, 27 Jun 2015 12:49:12 GMT express-session deprecated undefined saveUninitial
ized option; provide saveUninitialized option at app.js:20:9
moviesite started on port 3000

Sat, 27 Jun 2015 12:49:12 GMT express-session deprecated undefined resave option
; provide resave option at app.js:20:9
Sat, 27 Jun 2015 12:49:12 GMT express-session deprecated undefined saveUninitial
ized option; provide saveUninitialized option at app.js:20:9

如：
app.use(session({
resave: false,//重新保存：强制会话保存即使是未修改的。(默认值ture)
saveUninitialized: true,//强制保存未初始化的会话到存储器
cookie: {maxAge:3600000},
secret:'imooc',
store:new mongoStore({
url:dbUrl,
collection:'sessions'
})
}))