java - Hibernate 因 OneToOne 的多项选择而陷入困境

Question

我假设我遇到了 N+1 选择问题。

我得到了这个实体:

@Entity
@Table(name = "Devices")
public class Device implements Serializable {

 @OneToOne(mappedBy="holdingDevice", fetch=FetchType.LAZY)
    @Cascade(CascadeType.ALL)
    @PrimaryKeyJoinColumn
    private WarrantyEntry warranty;
}

这是另一个实体:

@Entity
@Table(name = "Warranty")
public class WarrantyEntry implements Serializable{
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;

    @OneToOne
    @JoinColumn(name = "serial")
    @PrimaryKeyJoinColumn
    private Device holdingDevice;

现在当我开始迭代这个循环时:

 Set<Device> customerDevices = user.getCustomer().getDevices();
            for (Device device : customerDevices) {
               ...
            }

我被卡住了，我在日志中看到 Hibernate 选择:

hibernate :*/选择warrantyen0_.id作为...Hibernate:/* 加载 myclass.Device */

hibernate :*/选择 Warrantyen0_.id 作为 ..Hibernate:/* 加载 myclass.Device */...

我一次又一次地猜想我遇到了 n+1 选择问题。

有什么建议我如何解决并用一个选项替换所有这些选择？

完成此操作后

 String query="from Customer c left join fetch c.devices d  \n" +
                    "left join fetch d.tradeInOldDevice "  +
                    "left join fetch d.tradeInNewDevice  "+
                    "left join fetch d.warranty";



            deviceDao.getSessionFactory().openSession().createQuery(query);


            for (Device device : customerDevices) {
               ..
            }

我仍然明白:

Hibernate: 
        devices0_.owningCompany_customerRefId as owningC15_0_1_,
        devices0_.serial as serial1_,
        devices0_.serial as serial9_0_,
        devices0_.blackListed as blackLis2_9_0_,
        devices0_.Creation_id as Creation12_9_0_,
        devices0_.deactivated as deactiva3_9_0_,
        devices0_.deviceComment as deviceCo4_9_0_,
        devices0_.deviceName as deviceName9_0_,
        devices0_.deviceType as deviceType9_0_,
        devices0_.distributor_customerRefId as distrib13_9_0_,
        devices0_.endCustomer_customerRefId as endCust14_9_0_,
        devices0_.owningCompany_customerRefId as owningC15_9_0_,
        devices0_.paChallenge as paChalle7_9_0_,
        devices0_.parent_serial as parent16_9_0_,
        devices0_.pendingDeactivation as pendingD8_9_0_,
        devices0_.safetyStock as safetySt9_9_0_,
        devices0_.serialSalt as serialSalt9_0_,
        devices0_.signedBlackBerry as signedB11_9_0_,
        devices0_.tradeInOldDevice as tradeIn17_9_0_ 
    from
        Devices devices0_ 
    where
        devices0_.owningCompany_customerRefId=?
Hibernate: 
        device0_.serial as serial9_0_,
        device0_.blackListed as blackLis2_9_0_,
        device0_.Creation_id as Creation12_9_0_,
        device0_.deactivated as deactiva3_9_0_,
        device0_.deviceComment as deviceCo4_9_0_,
        device0_.deviceName as deviceName9_0_,
        device0_.deviceType as deviceType9_0_,
        device0_.distributor_customerRefId as distrib13_9_0_,
        device0_.endCustomer_customerRefId as endCust14_9_0_,
        device0_.owningCompany_customerRefId as owningC15_9_0_,
        device0_.paChallenge as paChalle7_9_0_,
        device0_.parent_serial as parent16_9_0_,
        device0_.pendingDeactivation as pendingD8_9_0_,
        device0_.safetyStock as safetySt9_9_0_,
        device0_.serialSalt as serialSalt9_0_,
        device0_.signedBlackBerry as signedB11_9_0_,
        device0_.tradeInOldDevice as tradeIn17_9_0_ 
    from
        Devices device0_ 
    where
        device0_.tradeInOldDevice=?
Hibernate: 
        warrantyen0_.id as id34_2_,
        warrantyen0_.createdTime as createdT2_34_2_,
        warrantyen0_.deleted as deleted34_2_,
        warrantyen0_.expiryDate as expiryDate34_2_,
        warrantyen0_.serial as serial34_2_,
        warrantyen0_.updateTime as updateTime34_2_,
        warrantyen0_.updateUser as updateUser34_2_,
        device1_.serial as serial9_0_,
        device1_.blackListed as blackLis2_9_0_,
        device1_.Creation_id as Creation12_9_0_,
        device1_.deactivated as deactiva3_9_0_,
        device1_.deviceComment as deviceCo4_9_0_,
        device1_.deviceName as deviceName9_0_,
        device1_.deviceType as deviceType9_0_,
        device1_.distributor_customerRefId as distrib13_9_0_,
        device1_.endCustomer_customerRefId as endCust14_9_0_,
        device1_.owningCompany_customerRefId as owningC15_9_0_,
        device1_.paChallenge as paChalle7_9_0_,
        device1_.parent_serial as parent16_9_0_,
        device1_.pendingDeactivation as pendingD8_9_0_,
        device1_.safetyStock as safetySt9_9_0_,
        device1_.serialSalt as serialSalt9_0_,
        device1_.signedBlackBerry as signedB11_9_0_,
        device1_.tradeInOldDevice as tradeIn17_9_0_,
        management2_.id as id22_1_,
        management2_1_.deleted as deleted22_1_,
        management2_1_.firstName as firstName22_1_,
        management2_1_.lastLogin as lastLogin22_1_,
        management2_1_.lastName as lastName22_1_,
        management2_1_.password as password22_1_,
        management2_1_.primaryEmail as primaryE7_22_1_,
        management2_1_.userName as userName22_1_,
        management2_.authority as authority23_1_,
        management2_.isViewer as isViewer23_1_,
        management2_3_.distributor as distribu1_25_1_,
        management2_4_.umeKeysQuota as umeKeysQ1_27_1_,
        case 
            when management2_2_.id is not null then 2 
            when management2_3_.id is not null then 3 
            when management2_4_.id is not null then 5 
            when management2_5_.id is not null then 6 
            when management2_6_.id is not null then 7 
            when management2_.id is not null then 1 
        end as clazz_1_,
        cids3_.Users_id as Users1_22_4_,
        cids3_.element as element4_,
        emails4_.Users_id as Users1_22_5_,
        emails4_.element as element5_,
        roles5_.Users_Management_id as Users1_22_6_,
        roles5_.element as element6_ 
    from
        Warranty warrantyen0_ 
    left outer join
        Devices device1_ 
            on warrantyen0_.serial=device1_.serial 
    left outer join
        Users_Management management2_ 
            on warrantyen0_.updateUser=management2_.id 
    left outer join
        Users management2_1_ 
            on management2_.id=management2_1_.id 
    left outer join
        Users_Management_Administrators management2_2_ 
            on management2_.id=management2_2_.id 
    left outer join
        Users_Management_Distributors management2_3_ 
            on management2_.id=management2_3_.id 
    left outer join
        Users_Management_Limited management2_4_ 
            on management2_.id=management2_4_.id 
    left outer join
        Users_Management_Managers management2_5_ 
            on management2_.id=management2_5_.id 
    left outer join
        Users_Management_Workers management2_6_ 
            on management2_.id=management2_6_.id 
    left outer join
        Users_CID cids3_ 
            on management2_.id=cids3_.Users_id 
    left outer join
        Users_Emails emails4_ 
            on management2_.id=emails4_.Users_id 
    left outer join
        Users_Management_roles roles5_ 
            on management2_.id=roles5_.Users_Management_id 
    where
        warrantyen0_.serial=?

谢谢，射线。

Answer 1

假设您已在 Customer 类中定义了 customerDevices 关联，如下所示:

@OneToMany(fetch = FetchType.LAZY, mappedBy = "device")
private Set<Device> customerDevices;

这种LAZY映射关联会让您面临n+1选择问题。让我们考虑一个简单的查询，该查询检索给定 customerId 的 customer:

session().createQuery("from Customer c where c.name=:name").setParameter("name", name);

这将返回一个 customer，其中 customerDevices 的集合是未初始化的集合包装器。现在，当您迭代循环时:

Set<Device> customerDevices = user.getCustomer().getDevices();
            for (Device device : customerDevices) {
               ...
            }

在访问设备集合时，Hibernate 必须从数据库中获取这个惰性集合，执行额外的select语句。

针对此问题的推荐解决方案是在代码中覆盖运行时的默认获取策略，您可以通过使用如下查询来实现:

session().createQuery(from Customer c left join fetch c.devices d  
left join fetch d.warrantyEntry)

这将返回一个 Customer 以及关联的集合。因此，不要只检索初始查询中的顶级对象，而是通过准确指定哪些数据来获取初始查询中所有需要的数据。将在正在进行的工作单元中访问关联。

编辑:

看起来有时一对一映射会导致 HQL 急切获取失败。您可以在这些讨论中找到有关此问题的更多信息:HQL eager fetch failure和 N+1 selects on left join .

其中提到的一个解决方案是将 OneToOne 映射更改为 ManyToOne 和 OneToMany，我尝试过(使用类似类型的模型类) )并且它非常适合我。我通过一个 sql 查询获得了选择的所有结果。

例如，在 Device 类中，您可以将 OneToOne 映射更改为 ManyToOne:

@ManyToOne(cascade = CascadeType.ALL)
@JoinColumn(name = "warrantyFK", unique = true)
private WarrantyEntry warranty;

unique=true 属性将使此关联成为一对一关联，因为没有两个 Device 可以具有相同的 WarrantyEntry 。您可以在 WarrantyEntry 类中定义 Device 的集合:

@OneToMany(mappedBy = "warranty")
private Set<Device> holdingDevices;

主要思想是使用 OneToMany 和 ManyToOne 而不是 OneToOne。您只需确保最多向映射 OneToMany 一侧的 set 添加一项。