Create routes for adding a comment to an article

[vishwaefor]

Steps

POST /articles/:id/comments should be implemented like in above issues.

router.post(
  '/:id/comments',
  [
    // title should be given
    check('comment')
      .exists()
      .withMessage('comment is not provided')
      .isLength({ min: 10 })
      .withMessage('minimum 10 charactors required for comment')
  ],
  (req, res, next) => {
    Articles.findById(req.params.id)
      .populate('author', 'comments.author')
      .then(
        r => {
          if (r) {
            const errors = validationResult(req);

            if (!errors.isEmpty()) {
              res.status(422).json({ errors: errors.array() });
            } else {
              const comment = req.body;
              Users.findOne() // We are tempory adding a user as the author
                .then(author => {
                  comment.author = author._id;
                  Comments.create(comment)
                    .then(comment => {
                      r.comments.push(comment);
                      return r.save();
                    })
                    .then(r => {
                      res.status(201).json({ message: 'comment added' });
                    });
                });
            }
          } else {
            const error = new Error('no article found');
            error.status = 404;
            throw error;
          }
        },
        err => {
          const error = new Error('invalid article id');
          error.status = 400;
          throw error;
        }
      )

      .catch(err => next(err));
  }
);