One of the nicest things of GraphQL is the type system indeed. You can use tools like https://www.prisma.io/, which generates a GraphQL backend based on your SQL database.
Sorry for the cryptic error. That happens because the two objects in the comments array reference post_id. If you change those keys to book_id everything should work.
The algorithm for generating a GraphQL schema from a JSON object could be improved, along with its error handling. I use the algorithm from this repo: https://github.com/marmelab/graphql-schema-from-json. Error messages come from there.
I'll work on improving this tool and make it more user friendly. But I think that it is worth sharing it, even at this stage.
Congrats on shipping something so awesome in so little time!
Also thanks for this writeup. You recently migrated your servers to Kubernetes, right? What learning resources would you recommend for deploying a Phoenix app to k8?
If it is too time consuming to answer my questions, don't worry. The most important part of this message is thank you for building codesandbox. It makes the web a better place.