I had to map the query results to DTO and make it into json format, but since the structure was such that arrays within arrays were in the form of keys and values, I couldn't solve this problem even after struggling for 3 days. However, I carefully watched the collection lookup optimization lecture again and got hints, and I finally solved it. Watching the lecture again, I could see how much thought went into choosing the practical examples. Thank you again for creating a great lecture.