Schema has not been registered error on population

When I'm trying to populate , this error is coming: " MissingSchemaError: Schema hasn't been registered for model "

My models: /models/course.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var User = require('./user')

var courseSchema = Schema({
    courseName:String,
    price:Number,
    type:String,
    _offeredBy:{type:Schema.Types.ObjectId,ref:User}
});

module.exports = mongoose.model("Course",courseSchem 

models/user.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var bcrypt = require('bcrypt-nodejs');
var findOrCreate = require('mongoose-findorcreate')
var Course = require('./course')

var userSchema = Schema({
    email:{type:String},
    password:{type:String},
    courseOffered:[{type:Schema.Types.ObjectId,ref:Course}]
});


module.exports = mongoose.model('User',userSchema);

routes/index.js

var Course = require('../models/course');
var User = require('../models/user');
var express = require('express');
var router = express.Router();

router.get('/user/profile',isLoggedIn,function(req,res,next){
res.render('user/profile',{csrfToken:req.csrfToken(),message:""})
});
router.post('/user/profile',isLoggedIn,function(req,res,next){
var courseInfo = req.body;
var newCourse = new Course();
newCourse.courseName = courseInfo.coursename;
newCourse.price = courseInfo.price;
newCourse.type = courseInfo.type;
newCourse._offeredBy = req.user;
newCourse.save(function(err,result){
if(err){
  res.redirect('/user/profile');
}
});
Course
.findOne({courseName:courseInfo.coursename})
.populate('_offeredBy')
.exec(function(err,course){
if(err){
  res.redirect('/user/profile');
}
});

});

Course is getting saved in the database, but the error is coming due to pouplating. I'm not writing the app.js, mongodb connections are made in app.js file.

via ASHUTOSH CHANDRA