我正在编写一个JPQL查询(Hibernate作为我的JPA提供者(来获取实体Company
及其几个关联。这适用于我的"简单"ManyToMany关联,如下所示:
@Entity
@Table(name = "company")
@NamedQueries({
@NamedQuery(
name = "Company.profile.view.byId",
query = "SELECT c " +
"FROM Company AS c " +
"INNER JOIN FETCH c.city AS city " + <-- @ManyToOne
"LEFT JOIN FETCH c.acknowledgements " + <-- @ManyToMany
"LEFT JOIN FETCH c.industries " + <-- @ManyToMany
"WHERE c.id = :companyId"
)
})
public class Company { ... }
Hibernate创建一个查询来获取上述内容,这很好。但是,我的Company
实体也与存储在中间表中的数据具有多对多关联,因此将其映射为三个实体之间的@OneToMany
和@ManyToOne
关联。
公司<-- 公司服务 -->服务
这是我的代码中的三个实体。因此,Company
实例具有CompanyService
个实体的集合,每个实体都与Service
实例相关。我希望这是有道理的 - 否则请检查问题末尾的源代码。
现在我想通过修改上述查询来获取给定公司的服务。我事先读到JPA不允许嵌套的获取连接,甚至不允许连接的别名,但一些JPA提供者确实支持它,所以我尝试了Hibernate的运气。我尝试修改查询如下:
@Entity
@Table(name = "company")
@NamedQueries({
@NamedQuery(
name = "Company.profile.view.byId",
query = "SELECT c " +
"FROM Company AS c " +
"INNER JOIN FETCH c.city AS city " +
"LEFT JOIN FETCH c.acknowledgements " +
"LEFT JOIN FETCH c.industries " +
"LEFT JOIN FETCH c.companyServices AS companyService " +
"LEFT JOIN FETCH companyService.service AS service " +
"WHERE c.id = :companyId"
)
})
public class Company { ... }
现在,Hibernate不会创建单个查询,而是创建以下查询:
#1
select ...
from company company0_
inner join City city1_ on company0_.postal_code = city1_.postal_code
[...]
left outer join company_service companyser6_ on company0_.id = companyser6_.company_id
left outer join service service7_ on companyser6_.service_id = service7_.id
where company0_.id = ?
#2
select ...
from company company0_
inner join City city1_ on company0_.postal_code = city1_.postal_code
where company0_.id = ?
#3
select service0_.id as id1_14_0_, service0_.default_description as default_2_14_0_, service0_.name as name3_14_0_
from service service0_
where service0_.id = ?
#4
select service0_.id as id1_14_0_, service0_.default_description as default_2_14_0_, service0_.name as name3_14_0_
from service service0_
where service0_.id = ?
查询 #1我省略了不相关的连接,因为这些都可以。它似乎选择了我需要的所有数据,包括服务和中间实体数据(CompanyService
(。
查询 #2此查询只是从数据库及其City
中获取公司。城市关联被急切地获取,但即使我将其更改为延迟获取,查询仍然会生成。所以老实说,我不知道这个查询是干什么用的。
查询 #3 + 查询 #4这些查询基于 ID 查找Service
实例,大概基于查询 #1 中获取的服务 ID。我认为不需要此查询,因为此数据已在查询 #1 中获取(就像查询 #2 中的数据已在查询 #1 中获取一样(。此外,如果一家公司有许多服务,这种方法显然不能很好地扩展。
奇怪的是,查询 #1 似乎可以满足我的需求,或者至少它会获取我需要的数据。我只是不知道为什么Hibernate会创建查询#2,#3和#4。所以我有以下问题:
- 为什么 Hibernate 会创建查询 #2、#3 和 #4?我能避免吗?
- Hibernate是否支持嵌套关联获取,即使JPA不支持?如果是这样,在我的情况下我会怎么做?
- 这种行为是正常的,还是因为我尝试做的事情不受支持,因此我得到了奇怪的结果?这看起来很奇怪,因为查询 #1 看起来非常好
任何错误或替代解决方案的指示来完成我想要的事情将不胜感激。下面是我的代码(不包括getter和setter(。提前非常感谢!
公司实体
@Entity
@Table(name = "company")
@NamedQueries({
@NamedQuery(
name = "Company.profile.view.byId",
query = "SELECT c " +
"FROM Company AS c " +
"INNER JOIN FETCH c.city AS city " +
"LEFT JOIN FETCH c.acknowledgements " +
"LEFT JOIN FETCH c.industries " +
"LEFT JOIN FETCH c.companyServices AS companyService " +
"LEFT JOIN FETCH companyService.service AS service " +
"WHERE c.id = :companyId"
)
})
public class Company {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column
private int id;
// ...
@ManyToOne(fetch = FetchType.EAGER, targetEntity = City.class, optional = false)
@JoinColumn(name = "postal_code")
private City city;
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "company_acknowledgement", joinColumns = @JoinColumn(name = "company_id"), inverseJoinColumns = @JoinColumn(name = "acknowledgement_id"))
private Set<Acknowledgement> acknowledgements;
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "company_industry", joinColumns = @JoinColumn(name = "company_id"), inverseJoinColumns = @JoinColumn(name = "industry_id"))
private Set<Industry> industries;
@OneToMany(fetch = FetchType.LAZY, mappedBy = "company")
private Set<CompanyService> companyServices;
}
公司服务实体
@Entity
@Table(name = "company_service")
@IdClass(CompanyServicePK.class)
public class CompanyService implements Serializable {
@Id
@ManyToOne(targetEntity = Company.class)
@JoinColumn(name = "company_id")
private Company company;
@Id
@ManyToOne(targetEntity = Service.class)
@JoinColumn(name = "service_id")
private Service service;
@Column
private String description;
}
服务实体
@Entity
@Table(name = "service")
public class Service {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column
private int id;
@Column(length = 50, nullable = false)
private String name;
@Column(name = "default_description", nullable = false)
private String defaultDescription;
}
获取数据
public Company fetchTestCompany() {
TypedQuery<Company> query = this.getEntityManager().createNamedQuery("Company.profile.view.byId", Company.class);
query.setParameter("companyId", 123);
return query.getSingleResult();
}
好吧,看来我想通了。通过在CompanyService
中将获取类型设置为FetchType.LAZY
,Hibernate停止生成所有基本上再次获取相同数据的冗余查询。下面是实体的新版本:
@Entity
@Table(name = "company_service")
@IdClass(CompanyServicePK.class)
public class CompanyService implements Serializable {
@Id
@ManyToOne(fetch = FetchType.LAZY, targetEntity = Company.class)
@JoinColumn(name = "company_id")
private Company company;
@Id
@ManyToOne(fetch = FetchType.LAZY, targetEntity = Service.class)
@JoinColumn(name = "service_id")
private Service service;
@Column
private String description;
}
JPQL 查询保持不变。
但是,在我的特殊情况下,我的Company
实体具有的关联数量,我得到了大量重复的数据,因此让Hibernate执行额外的查询更有效。我通过从 JPQL 查询中删除两个连接提取并将我的查询代码更改为以下内容来实现此目的。
@Transactional
public Company fetchTestCompany() {
TypedQuery<Company> query = this.getEntityManager().createNamedQuery("Company.profile.view.byId", Company.class);
query.setParameter("companyId", 123);
try {
Company company = query.getSingleResult();
Hibernate.initialize(company.getCompanyServices());
return company;
} catch (NoResultException nre) {
return null;
}
}
通过初始化companyServices
关联,Hibernate执行另一个查询来获取服务。在我的特定用例中,这比使用一个查询获取大量冗余数据要好。
我希望这对某人有所帮助。如果有人有任何更好的解决方案/改进,那么我当然很乐意听到他们。
从你写的内容来看,我会说不支持嵌套获取。这是我对您的结果的理解:
- 查询#1没问题,并连接了它需要的一切,这很好
- 但是,我认为查询#2会
CompanyService#company
(急切city
导致inner join City
( -
查询 #3 获取
CompanyService#service
- 查询#4对我来说是一个迷雾
我知道这不是一个答案,但它可能会帮助你了解后台发生的事情。