Many to One Query in jpa springboot

 Many to One Query in jpa springboot

In JPA and Spring Boot, a "many to one" relationship is a type of entity relationship where multiple instances of one entity are related to a single instance of another entity. For example, a customer can have many orders, but each order is associated with only one customer.

To perform a "many to one" query in JPA and Spring Boot, you can use the @ManyToOne annotation to define the relationship between entities. Then, you can use a JOIN clause in your JPQL query to fetch data from both entities.

For example, let's say we have two entities: Order and Customer. The Order entity has a many-to-one relationship with the Customer entity, as follows:

java
@Entity
@Table(name = "orders")
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "customer_id")
    private Customer customer;

    // other fields, getters and setters
}

java
@Entity
@Table(name = "customers")
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // other fields, getters and setters
}

To fetch all orders for a specific customer, you can use a JOIN query like this:

java
public interface OrderRepository extends JpaRepository<Order, Long> {
    @Query("SELECT o FROM Order o JOIN o.customer c WHERE c.id = :customerId")
    List<Order> findByCustomerId(Long customerId);
}