async posts(postId) {
      try {
        if (this.favorites.posts.indexOf(postId) >= 0) {
          this.favorites.posts.remove(postId);
          await Post.decFavoriteCount(postId);
        } else {
          await Post.incFavoriteCount(postId);
          this.favorites.posts.push(postId);
        }

        return this.save();
      } catch (err) {
        return err;
      }
    }
Favorite a post or unfavorite if already here

@param {String} postId - _id of the post like
@returns {Promise}