Hello teacher. I came back from a wild trip, so I took a lecture after a long time. When I was rolling around in the wild, the thing that bothered me the most was DB.. In particular, the most difficult thing to solve and understand was transaction. Why doesn't this work? Why does this work? It was the main character. It was especially confusing because it was mixed with association mapping. But while listening to your lecture, I finally got a clue as to why the error I experienced occurred, and why it was solved by adding @Transactional. Thank you for always giving great lectures!