如何查询外键字符串ID而不获取实际实体？

可以使用JPA的元模型类（metamodel）以及JPQL语句来实现此目的。假设有以下实体类：

@Entity
public class Order {
    @Id
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private Customer customer;

    // other fields and methods
}

@Entity
public class Customer {
    @Id
    private Long id;

    private String name;

    // other fields and methods
}

我们可以先生成对应的元模型类：

@StaticMetamodel(Order.class)
public class Order_ {
    public static volatile SingularAttribute<Order, Long> id;
    public static volatile SingularAttribute<Order, Customer> customer;
}

@StaticMetamodel(Customer.class)
public class Customer_ {
    public static volatile SingularAttribute<Customer, Long> id;
    public static volatile SingularAttribute<Customer, String> name;
}

然后可以在JPQL语句中使用这些元模型属性来查询外键字符串ID：

public interface OrderRepository extends JpaRepository<Order, Long> {
    @Query("SELECT o.customer.id FROM Order o WHERE o.id = :orderId")
    String findCustomerIdByOrderId(@Param("orderId") Long orderId);
}

这样，调用findCustomerIdByOrderId方法时，就可以只获取外键字符串ID而不获取实际的Customer实体了。