how do i push a document on a nested mongodb - mongodb

Question

if I want to add a review, with author,rating and reviewText to the first title, how do I do it.
{
"_id": ObjectId("58dd21c3cb77090b930b6063"),
"bookAuthor": "wwww",
"titles": [{
"title": "this is title",
"_id": ObjectId("58dd3f2701cc081056135dae"),
"reviews": [],
"favouredBy": [
"bb, aa, cc"
]
}, {
"title": "this is the second tittle",
"_id": ObjectId("58dd42a59f12f110d1756f08"),
"reviews": [],
"favouredBy": [
"all"
]
}],
"__v": 0
}
from with in the application this is what I tried but keep getting "TypeError: Cannot read property 'push' of undefined" error
module.exports.reviewsCreate = function(req, res) {
if (req.params.bookid) {
Bok
.findById(req.params.bookid)
.select('titles')
.exec(
function(err, book) {
if (err) {
sendJSONresponse(res, 400, err);
} else {
doAddReview(req, res, book);
}
}
);
} else {
sendJSONresponse(res, 404, {
"message": "Not found, bookid required"
});
}
};
var doAddReview = function(req, res, book, author) {
console.log(book);
if (!book) {
sendJSONresponse(res, 404, "bookid not found");
} else {
book.titles.reviews.push({
author: author,
rating: req.query.rating,
reviewText: req.query.reviewText
});
book.save(function(err, book) {
var thisReview;
if (err) {
sendJSONresponse(res, 400, err);
} else {
sendJSONresponse(res, 201, book);
}
});
}
};