Hibernate 5.4: How to Handle Bidirectional Relationships with Mixed Fetch Strategies and Avoid N+1 Queries?
I've been banging my head against this for hours... I'm working with a scenario with Hibernate 5.4 regarding bidirectional relationships in my JPA entities. I have a `User` entity that has a `OneToMany` relationship with an `Order` entity. The `Order` entity has a `ManyToOne` relationship back to `User`. My goal is to efficiently fetch users along with their associated orders without running into N+1 query issues, but I'm noticing a important performance hit due to lazy loading behavior. Here are the relevant snippets of my entities: ```java @Entity public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @OneToMany(mappedBy = "user", fetch = FetchType.LAZY) private List<Order> orders; } @Entity public class Order { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; @ManyToOne(fetch = FetchType.EAGER) @JoinColumn(name = "user_id") private User user; } ``` When I try to fetch users along with their orders, I call: ```java List<User> users = entityManager.createQuery("SELECT u FROM User u", User.class).getResultList(); ``` This leads to multiple queries being executed, especially if I have a large number of users, which results in the dreaded N+1 question. To mitigate this, I've tried using a `JOIN FETCH` in my query: ```java List<User> users = entityManager.createQuery("SELECT u FROM User u JOIN FETCH u.orders", User.class).getResultList(); ``` However, this approach leads to a `NonUniqueResultException` when I have a user with multiple orders. I also looked into the option of using `@BatchSize` on the `orders` collection, but I'm unsure how it might affect my fetch strategy. What is the best way to handle this situation, ensuring I fetch users and their orders efficiently without hitting performance optimization or running into exceptions? Any suggestions on optimizing the fetch strategies or restructuring my queries would be greatly appreciated! What am I doing wrong?