updating a post

update-post.js

exports.update = function (req, res) {
  const newAuthorId = req.params.authorId;
  const { postId } = req.params;
  const newPost = req.body;

  PostModel.findOne({ _id: postId }, (err, post) => {
    if (!post) {
      return err;
    }

    const oldAuthorID = post.author._id;

    AuthorModel.findById(oldAuthorID)
      .then((oldAuthor) => {
        if (!oldAuthor) {
          return res.status(400).send('No Author with that Particular id');
        }

        const index = oldAuthor.posts.indexOf(postId);

        if (index > -1) {
          oldAuthor.posts.splice(index, 1);
        }

        oldAuthor.save((error, savedAuthor) => {
          if (error) {
            return error;
          }
          return savedAuthor;
        });
        return oldAuthor;
      });


    AuthorModel.findById(newAuthorId)
      .then((newAuthor) => {
        if (!newAuthor) {
          return err;
        }

        newAuthor.posts.push(post);
        newAuthor.save((error, savedAuthor) => {
          if (error) {
            return error;
          }
          return savedAuthor;
        });

        post.author = newAuthor;

        _.merge(post, newPost);
        post.save((error, saved) => {
          if (error) {
            return error;
          }
          return res.json(saved);
        });
        return newAuthor;
      });
    return post;
  });
};