How to use mongoose

static async getBySlug({ slug }) {
  const bookDoc = await this.findOne({ slug });
  if (!bookDoc) {
   throw new Error('Book not found');
  }

  const book = bookDoc.toObject();

  book.chapters = (await Chapter.find({ bookId: book._id }, 'title slug').sort({ order: 1 })).map(
   (chapter) => chapter.toObject(),
  );
  return book;
 }

router.get('/my-books', async (req, res) => {
 try {
  const { purchasedBookIds = [] } = req.user;

  const { purchasedBooks } = await Book.getPurchasedBooks({ purchasedBookIds });

  res.json({ purchasedBooks });
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

router.post('/books/add', async (req, res) => {
 try {
  const book = await Book.add(Object.assign({ userId: req.user.id }, req.body));
  res.json(book);
 } catch (err) {
  logger.error(err);
  res.json({ error: err.message || err.toString() });
 }
});

router.get('/books/detail/:slug', async (req, res) => {
 try {
  const book = await Book.getBySlug({ slug: req.params.slug });
  res.json(book);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

async function getEmailTemplate(name, params) {
 const source = await EmailTemplate.findOne({ name });
 if (!source) {
  throw new Error(`No EmailTemplates found.
   Please check that at least one is generated at server startup,
   restart your server and try again.`);
 }

 return {
  message: _.template(source.message)(params),
  subject: _.template(source.subject)(params),
 };
}

router.get('/books', async (req, res) => {
 try {
  const books = await Book.list();
  res.json(books);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

router.get('/books/:slug', async (req, res) => {
 try {
  const book = await Book.getBySlug({ slug: req.params.slug, userId: req.user && req.user.id });
  res.json(book);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

router.post('/books/edit', async (req, res) => {
 try {
  const editedBook = await Book.edit(req.body);
  res.json(editedBook);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

router.post('/books/edit', async (req, res) => {
 try {
  const editedBook = await Book.edit(req.body);
  res.json(editedBook);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

router.get('/books', async (req, res) => {
 try {
  const books = await Book.list();
  res.json(books);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

async function getEmailTemplate(name, params) {
 const source = await EmailTemplate.findOne({ name });
 if (!source) {
  throw new Error(`No EmailTemplates found.
   Please check that at least one is generated at server startup,
   restart your server and try again.`);
 }

 return {
  message: _.template(source.message)(params),
  subject: _.template(source.subject)(params),
 };
}

router.get('/books', async (req, res) => {
 try {
  const books = await Book.list();
  res.json(books);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

static async getBySlug({ slug }) {
  const bookDoc = await this.findOne({ slug });
  if (!bookDoc) {
   throw new Error('Book not found');
  }

  const book = bookDoc.toObject();

  book.chapters = (await Chapter.find({ bookId: book._id }, 'title slug').sort({ order: 1 })).map(
   (chapter) => chapter.toObject(),
  );
  return book;
 }

router.get('/books', async (req, res) => {
 try {
  const books = await Book.list();
  res.json(books);
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});

router.post('/books/edit', async (req, res) => {
 try {
  await Book.edit(req.body);
  res.json({ done: 1 });
 } catch (err) {
  res.json({ error: err.message || err.toString() });
 }
});