Mongoose Query Data in Express.js Node.js ,get the wrong result

The Problem is .. sometimes it shows up the wrong user on the screen (someone gets session of another one). but it's hardly happen and i think it only happen when there're some concurrent.

if anything in this code can make this behaviour happen , please suggest.

app.js -- this file has a schema and initiation of model and routes component

var userSchema = mongoose.Schema({
    "_id": mongoose.Schema.Types.ObjectId,
    "name": String,
    "username":String,
    "etc":String
});
userMongooseModel = mongoose.model('users',userSchema);

var sessionSchema = mongoose.Schema({
    "userID": mongoose.Schema.Types.ObjectId,
    "sessionID": String,
    "expire":Number
}); 
sessionMongooseModel = mongoose.model('sessions',sessionSchema);

var UserModel = require(basePath+'/models/UserModel.js').UserModel;  
userModel = new UserModel();

var user = require(basePath+'/routes/user');

routes/user.js -- this file is the detail about each route.

exports.editProfilePage = function(req,res){
    var httpRes = res;
    userModel.checkSession(req.cookies.session,function(res){
        if(res.status=='success' && res.type=='photographer')
        {
            userModel.getByID(res.userID,{},function(resp){

                httpRes.render(basePath+'/views/photographer-edit.html',{currentUser:res.user,user:resp.user,etc:'etc'});
            });
        }
        else
        {
            //if not login or state != 0 
            httpRes.redirect(baseURL+'/photographerRedirect');
        }
    });
}

usermodel.js -- this file is to retrieve data from database

var mongoose = require('mongoose');
var ObjectId = mongoose.Types.ObjectId;
var request = require('request');

UserModel.prototype.checkSession = function(sessionID,callback){
    sessionMongooseModel.findOne({sessionID:sessionID},function (err, user) {       
        if(err)
        {
            callback({status:'fail',errorMsg:'notFound'});
            return;
        }
        if(user==null)
        {
            callback({status:'fail',errorMsg:'notFound'});
        }
        else
        {
            if(user.expire > Date.now())
            {
                userMongooseModel.findOne({_id:user.userID},{studioName:1,state:1,etc:1},function (err, user) {
                    if(err || user==null)
                    {
                        callback({status:'fail',errorMsg:'notFound'});
                        return;
                    }
                    if(user.type=='photographer' && user.state==0)
                    {
                        callback({status:'fail',errorMsg:'wrongUserState',userID:user._id,user:user,etc:1});
                    }
                    else
                        callback({status:'success',userID:user._id,user:user,type:user.type,etc:1});
                });             
            }
            else
            {
                callback({status:'fail',errorMsg:'notFound'});
            }
        }
    });
}


UserModel.prototype.getByIDs = function(userIDs,options,callback){
    userMongooseModel.find({_id:{$in:userIDs}},options,function (err, users) {
        if(err){
            callback({status:'fail',errorMsg:'UserModel.find'});
            return;
        }
        callback({status:'success',users:users});
    });
}

Thanks a lot !