我无法为profile创建索引:varuser=newSchema({profile:"String",fullname:"String"})user.statics.createIndexOfProfile=function(callback){this.ensureIndex("profile",function(err,doc){if(err){console.log(err+'sna');callback(err);}else{console.log(doc+'santhosh');callback(null,doc);}});我收到类似this.ensureIndexisno