Why isn't commentCount being updated in my test?

Question

I have a Post model and a Comment model. When a user posts a comment, the postID will be sent as a param.

Once the comment is saved, it's id will be pushed into the comments array that belongs to the post.

The Post model has a commentCount which is a virtual property, which will count how many comment IDs are in the comments array.

This is my commentController:

  create(req, res, next) {
    const commentProps = req.body;
    const postId = req.params.id;

    Comment.create(commentProps)
      .then(comment => {
        Post.findById({ _id: postId })
          .then(post => {
            console.log(post);
            console.log('comment id is ' + comment._id);

            post.comments.push(comment._id);

            console.log(post);
            console.log('commentCount is ' + post.commentCount);
          });
        return res.send(comment);
      })
      .catch(next);
  },

All the logs are coming through just as they should be. The comment._id gets pushed right into the comments array and the commentCount has been incremented by 1.

Yet my test assertion is not passing:

it('POST to /api/comments/:id creates a new comment and increases posts commentCount', done => {
    const post = new Post({
      author: user._id,
      text: 'This is a post',
      createdAt: 0,
      expiresAt: 0,
      voteCount: 0
    });

    post.save()
      .then(() => {
        Comment.count().then(count => {
          request(app)
            .post(`/api/comments/${post._id}`)
            .send({
              author: user._id,
              text: 'This is a comment',
              createdAt: 0,
              postId: 'randomPost'
            })
            .end(() => {
              Comment.count().then(newCount => {
                console.log('Comment count is ' + post.commentCount);

                assert(count + 1 === newCount);
                assert(post.commentCount === 1);

                done();
              });
            });
        });
      });
  });

The log in the test above comes back as 0 and if I log the post.comments then it just comes back as an empty array.

Any idea what I'm doing wrong here guys?

Thanks!