Hey,
One important thing in setting the code up above is to make sure you follow the guide the OP posted. You want to make sure that any ratings you capture for each user are collected into a separate collection, so that my rating doesn’t override your rating when we submit per dynamic page.
Another approach is to store all the ratings in a separate database, and then use the aggregate - Velo API Reference - Wix.com functionality to calculate the averages, total and number of ratings only when the page loads, rather than recalculating for every (movie/song/info in your DB.). It protects the DB from bad re-writes.
If you’re taking your dynamic pages beyond just ratings, also check out the Comments - Velo API Reference - Wix.com reference for how to create comments that are tied to that dynamic page.