Joining Unrelated Entities with JPA and Hibernate in Spring Boot

In this tutorial, we will explore how to join unrelated entities using JPA (Java Persistence API) and Hibernate in a Spring Boot application. The process of joining unrelated entities allows us to retrieve data from different tables that have no direct association using a single database query. By the end of this tutorial, you will have a better understanding of how to leverage JPA and Hibernate to perform complex queries efficiently.

Before proceeding with this tutorial, make sure you have the following in place:

1. Basic knowledge of Java and Spring Boot.
2. Familiarity with JPA and Hibernate concepts.
3. A working Spring Boot project set up with a relational database.

Define Entities

Let’s start by defining two unrelated entities that we want to join later on. For this tutorial, we’ll assume you have two entities: Product and Category. The Product entity represents products in an online store, while the Category entity represents product categories.

Product.java

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    // Other attributes, getters, and setters
}

Category.java

@Entity
public class Category {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    // Other attributes, getters, and setters
}

Create Repositories

Next, create JPA repositories for both entities to interact with the database. Each repository should extend the JpaRepository interface provided by Spring Data JPA.

ProductRepository.java

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
    // Custom queries, if needed
}

CategoryRepository.java

@Repository
public interface CategoryRepository extends JpaRepository<Category, Long> {
    // Custom queries, if needed
}

Implement the Query

To join unrelated entities, we’ll use the Hibernate CriteriaBuilder and CriteriaQuery. These APIs enable us to create complex queries programmatically.

In this example, we’ll join the Product and Category entities based on a common attribute, such as name. In the real world, you may need to join entities based on other attributes, so feel free to adjust the query accordingly.

@Service
public class ProductService {
    @Autowired
    private ProductRepository productRepository;

    @Autowired
    private CategoryRepository categoryRepository;

    public List<Product> getProductsByCategoryName(String categoryName) {
        CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
        CriteriaQuery<Product> criteriaQuery = criteriaBuilder.createQuery(Product.class);
        Root<Product> productRoot = criteriaQuery.from(Product.class);
        Join<Product, Category> categoryJoin = productRoot.join("category", JoinType.INNER);

        Predicate categoryPredicate = criteriaBuilder.equal(categoryJoin.get("name"), categoryName);
        criteriaQuery.where(categoryPredicate);

        TypedQuery<Product> typedQuery = entityManager.createQuery(criteriaQuery);
        return typedQuery.getResultList();
    }
}

Test the Query

With the query implementation in place, you can now test it using a controller or any other method of your choice.

@RestController
public class ProductController {
    @Autowired
    private ProductService productService;

    @GetMapping("/products-by-category/{categoryName}")
    public List<Product> getProductsByCategory(@PathVariable String categoryName) {
        return productService.getProductsByCategoryName(categoryName);
    }
}

Conclusion

In this tutorial, we learned how to join unrelated entities using JPA and Hibernate in a Spring Boot application. By leveraging the power of the Hibernate CriteriaBuilder and CriteriaQuery, we were able to perform complex queries efficiently. Remember to customize the join conditions based on your specific use case, and feel free to explore more advanced features provided by JPA and Hibernate to further enhance your application’s performance and data retrieval capabilities. Happy coding!