function addComment(comment) {
  return fetchPostBySlug(comment.postSlug).then((post) => {
    if (!post.isCommentingEnabled) {
      return Promise.reject(new Error('Commenting is disabled.'));
    }

    return isUserAllowedToComment(post, comment.user).then((isAllowed) => {
      if (!isAllowed) {
        return Promise.reject(
          new Error('The user is not allowed to comment on this post.')
        );
      }

      return insertComment(post.id, comment.user, comment.message).then((result) => {
        return result.record;
      })
    });
  })
}