mongoose + nodejs push array to database - arrays

Question

I'm new to NodeJS + Mongoose and having trouble pushing an array to my database via mongoose.
I have the following schema:
const StudentSchema = mongoose.Schema({
name: {
type: String
},
quizzes: [{
quiz: String,
answers: []
}]
});
What I'm trying to do is have an array of quiz objects in which the quiz number is shown and the array of answers for that particular quiz.
When I create a new user, the student object looks like this in mongoose:
{
"_id" : ObjectId(id here),
"name" : "John",
"quizzes" : [ ],
"__v" : 0
}
The function I'm using to update the quiz array:
module.exports.addQuiz = function(student_id, answers, callback){
Student.findByIdAndUpdate(
student_id,
{$push: {"quizzes": {answers: answers}}},
{safe: true, upsert: true},
function(err, model) {
console.log(err);
}
);
}
And this is my route which calls the function whenever the endpoint is hit with a student_id, which will then be used to find the student and push to the array
router.post('/quiz/:student_id', (req, res, next) => {
var student_id = req.params.id;
var answers = req.body.answers;
Student.addQuiz(student_id, answers, (err, answers) => {
//error handling
})
});
I'm trying to test this by sending a post request to /quiz/:student_id with an id of a student in my database with the following JSON sent in the body:
[{
"quiz": "Quiz 1",
"answers": ["Answer 1", "Answer 2"]
}]
Although when I try this it ends up getting hung somewhere and the request never completes - I also get a "null" in the console.
Can anyone help me out? Thank you.