Mongoose populate multiple nested documents

An interest alternative is pass an array to nested populate.

Course
    .findById(req.params.courseId)
    .populate({
        path: 'classes',
        model: 'Classroom',
        populate: [{
            path: 'instructors',
            model: 'User'
        },
        {
            path: 'location',
            model: 'Location'
        }]
    })

Please try this one under mongoose v4, here is one good link about Population

   Course
        .findById(req.params.courseId)
        .populate({
            path: 'classes',
            model: 'Classroom',
            populate: {
                path: 'instructors',
                model: 'User'           
            }
        })
        .exec(function(err, cour) {
            if (err)
                console.log(err);
            else {
                Course.populate(cour, 
                    {
                        path: 'classes.location',
                        model: 'Location',
                    }, function(err, c1) {
                        if (err)
                            console.log(err);
                        else
                            console.log(util.inspect(c1, { showHidden: true, depth: null }));                       
                    })
            }
        })