Eager fetch 在休眠中执行左连接，但在 springboot/JPA 中触发单独的 sql 查询

Question

我看到很多帖子，其中 Eager fetch 在休眠中执行子表父表的左连接。但是当我使用 springboot 时，hibernate 会触发单独的 sql 查询 - 意味着父表的一个选择查询和子表的一个选择查询。为什么有区别？ springboot有什么升级还是我做错了什么？

以下是我正在使用的实体：

订单实体：

@Entity
@Table(name="Ordertable", schema="cf_2583f365_c3c6_499a_a60d_138e7e7023eb")

public class Order {

@Id
@Column(name = "ORDER_ID")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int orderId;


@Column(name = "DAT_SRC_ID")
private String dataSourceId;

@ManyToOne(fetch=FetchType.EAGER)
@JoinColumn(name = "ORDER_CUSTOMER_ID", referencedColumnName = "CUSTOMER_ID") 
private Customer customer;
}

客户实体：

@Entity
@Table(name="Customer", schema="cf_2583f365_c3c6_499a_a60d_138e7e7023eb")
public class Customer {

    @Id
    @Column(name = "CUSTOMER_ID")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long customerId;

    @Column(name = "CUSTOMER_NAME")
    private String customer_name;


    @Column(name = "CUSTOMER_address_id")
    private int customer_address_id;

    @Column(name = "DAT_SRC_ID")
    private String dataSourceId;

    @OneToMany(fetch=FetchType.EAGER)
    @JoinColumn(name = "ORDER_CUSTOMER_ID", referencedColumnName = "CUSTOMER_ID") 
    private List<Order> order;

}

控制器：

@RequestMapping(value="/getByCustid/{id}", method=RequestMethod.GET,produces=MediaType.APPLICATION_JSON_VALUE)
public Customer getByCustid (@PathVariable Long id) {

    Customer s1 = customerRepository.findByCustomerId(id);
    return s1;
}

存储库：

public interface CustomerRepository extends JpaRepository<Customer,Long> {

    public Customer findByCustomerId(Long customerId); 

}

以下是正在执行的查询：

select
    customer0_.CUSTOMER_ID as CUSTOMER1_0_,
    customer0_.CUSTOMER_address_id as CUSTOMER2_0_,
    customer0_.CUSTOMER_NAME as CUSTOMER3_0_,
    customer0_.DAT_SRC_ID as DAT_SRC_4_0_ 
from
    Customer customer0_ 
where
    customer0_.CUSTOMER_ID=?

select
    order0_.ORDER_CUSTOMER_ID as ORDER_CU3_5_0_,
    order0_.ORDER_ID as ORDER_ID1_5_0_,
    order0_.ORDER_ID as ORDER_ID1_5_1_,
    order0_.ORDER_CUSTOMER_ID as ORDER_CU3_5_1_,
    order0_.DAT_SRC_ID as DAT_SRC_2_5_1_ 
from
    Ordertable order0_ 
where
    order0_.ORDER_CUSTOMER_ID=?

编辑：这是否与@Fetch(FetchMode.JOIN) 相关——链接：JPA eager fetch does not join

Hibernate: Multiple select queries made by Hibernate for Fetch mode Eager

为了检查 FetchMode.JOIN 是否有效，我在实体中添加了 FetchMode.JOIN，如下所示，但加入查询仍然没有成功：

客户实体：

@Entity
@Table(name="Customer", schema="cf_2583f365_c3c6_499a_a60d_138e7e7023eb")
public class Customer {
    @OneToMany(fetch=FetchType.EAGER)
    @JoinColumn(name = "ORDER_CUSTOMER_ID", referencedColumnName = "CUSTOMER_ID")
    @Fetch(FetchMode.JOIN)
    private List<Order> order;
}

订单实体：

@Entity
@Table(name="Ordertable", schema="cf_2583f365_c3c6_499a_a60d_138e7e7023eb")
public class Order {
    @JsonIgnore
    @ManyToOne()
    @JoinColumn(name = "ORDER_CUSTOMER_ID", referencedColumnName = "CUSTOMER_ID") 
    @Fetch(FetchMode.JOIN)
    private Customer customer;
}

Answer 1

findByCustomerId 实际上会根据该方法生成查询，而不是使用em.find。它将创建类似于SELECT c FROM Customer c WHERE c.customerId=:customerId 的东西。之后它会注意到 fetch 策略并获取所需的引用。这也解释了here。查询将完全按照您的指示执行。

如果您想立即加载参考，您需要自己按照SELECT c FROM Customer c JOIN FETCH c.orders o WHERE c.customerId=:customerId 的行编写查询，这将自动检索订单。

但是 customerId 实际上是您的实体的主键或标识符，因此您实际上应该使用 findById 或 findOne 方法（取决于您的 Spring Data JPA 版本）。这将使用EntityManager.find，它应该考虑映射信息并创建适当的查询。