node.js连接mongodb数据库警告：（Please ensure that you set the default write concern）

启动node项目服务时出现：

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 acknowlegement                                                       =
=     journal: true/false, wait for flush to journal before acknowlegement             =
=     fsync: true/false, wait for flush to file system before acknowlegement           =
=                                                                                      =
=  For backward compatibility safe is still supported and                              =
=   allows values of [true | false | {j:true} | {w:n, wtimeout:n} | {fsync:true}]      =
=   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 acknowlegement will change in the very near future                =
=                                                                                      =
=  This message will disappear when the default safe is set on the driver Db           =
========================================================================================

的信息。虽然不影响测试但是还是存在安全问题的。

解决方法：

将数据的连接设置为安全的连接（在构建数据对象的地方）：

module.exports = new Db(settings.db, new Server(settings.host, Connection.DEFAULT_PORT, {safe:true}),{safe:true});

添加上边红色字体可以解决。