Refactor and fix JPA code

[binkoni]

https://www.jpa-buddy.com/blog/best-practices-and-common-pitfalls/
https://blog.sapzil.org/public/springcamp2019.pdf

https://stackoverflow.com/questions/7197181/jpa-unidirectional-many-to-one-and-cascading-delete

https://www.inflearn.com/questions/39769

https://madplay.github.io/post/avoid-n+1-problem-in-jpa-using-querydsl-fetchjoin

https://stackoverflow.com/questions/27106274/jpa-hibernate-on-delete-set-null

https://kapentaz.github.io/jpa/hibernate/@ManyToOne%EC%9D%98-N+1-%EB%AC%B8%EC%A0%9C-%EC%9B%90%EC%9D%B8-%EB%B0%8F-%ED%95%B4%EA%B2%B0/#

https://dabok407.tistory.com/73

Setup cascade properly

https://www.baeldung.com/jpa-cascade-remove-vs-orphanremoval
https://stackoverflow.com/questions/14875793/jpa-hibernate-how-to-define-a-constraint-having-on-delete-cascade
https://thorben-janssen.com/avoid-cascadetype-delete-many-assocations/
https://www.bezkoder.com/spring-boot-unit-test-jpa-repo-datajpatest/

Orphan Removal

You can use in a parent entity, when you delete children from the list, then the children records are actually deleted from the database by JPA using genrated queries.
Currently there are no cases for Orphan Removal

ON DELETE CASCADE

You can use in a child entity. it affects the actual generated schema. When you delete parent entity, then child entities are automatically deleted by the database
Applied ON DELETE CASCADE

ON DELETE SET NULL

http://www.yannlaviolette.com/2013/09/jpa-hibernate-on-delete-set-null.html
Only @PreRemove approach is available
but if you don't want to use @onetomany you just have set null manually
Decided to use the manual approach

Use Transactional annotation properly

https://www.podo-dev.com/blogs/285
https://velog.io/@ddongh1122/Spring-Transactional-%ED%81%B4%EB%9E%98%EC%8A%A4-%EB%82%B4%EB%B6%80-%ED%98%B8%EC%B6%9C-%EB%AF%B8%EC%9E%91%EB%8F%99-%EC%9D%B4%EC%8A%88
https://stackoverflow.com/questions/4396284/does-spring-transactional-attribute-work-on-a-private-method
https://medium.com/javarevisited/spring-transactional-mistakes-everyone-did-31418e5a6d6b

Handle JPA errors properly

https://stackoverflow.com/questions/52456783/cannot-catch-dataintegrityviolationexception
https://www.baeldung.com/hibernate-initialize-proxy-exception
https://www.podo-dev.com/blogs/278
https://stackoverflow.com/questions/50150647/handling-exceptions-during-a-transactional-method-in-spring
Use approaches like AOP, inheritance, composition or callback methods
Decided to use AOP approach

TODOs

• Add ON DELETE CASCADE
• Add ON DELETE SET NULL
• Use FetchType.LAZY for ManyToOne, OneToOne
• Solve N+1 problem (with querydsl fetch join)
• Use MappedSuperclass or Embedded annotation Use Entity inheritance
• Use Transactional annotation properly (use only when needed, and use readonly transactional annotation)
• Handle JPA errors properly
• Use batch update/insert operations when needed
• (not sure) Consider using spring data jpa Domain specifications (Specification) | QueryDslPredicateExecutor | QueryDslRepositorySupport