Cascade Pitfalls

If a child already exists in the database but the parent cascades PERSIST to it, Hibernate tries to insert it again:

Category electronics = new Category();
electronics.setId(3L);                     // existing row, but this object is not managed
product.setCategory(electronics);
entityManager.persist(product);            // with cascade = PERSIST on Product.category

jakarta.persistence.EntityExistsException: detached entity passed to persist: com.shopease.catalog.Category

Fix: remove the cascade on @ManyToOne, and get a reference to the existing row with categoryRepository.getReferenceById(3L) (no SELECT, just a proxy with the id).

Cascade REMOVE loads every child and deletes them one by one. Deleting a category with 20,000 products would run 20,000 DELETE statements. For bulk deletes use a JPQL bulk statement instead (it skips cascades and the persistence context):

@Modifying
@Query("delete from OrderItem i where i.order.id = :orderId")
int deleteItemsOfOrder(@Param("orderId") Long orderId);

• Cascade only from aggregate root to its own children (Order → OrderItem, Product → ProductImage).
• No cascade on @ManyToOne; never REMOVE on @ManyToMany.
• Use orphanRemoval when "removed from the list" should mean "deleted".
• Use bulk JPQL or native deletes for large data sets.