使用主键连接列的一对一映射

Question

我使用主键连接列 (employee_id) 映射了员工和员工详细信息类（双向）

@Entity
@Table(name="employee")
public class Employee {

@Id
@GeneratedValue
@Column(name="employee_id")
private Long employeeId;

@Column(name="firstname")
private String firstname;

@Column(name="lastname")
private String lastname;

@Column(name="birth_date")
@Temporal(value = TemporalType.DATE )
private Date birthDate;

@Column(name="cell_phone")
private String cellphone;

@OneToOne(mappedBy="empl", cascade=CascadeType.ALL)
private EmployeeDetail employeeDetail;

...
}



@Table(name="employeedetail")
public class EmployeeDetail {


@Id
@Column(name="employee_id", unique=true, nullable=false)
@GeneratedValue(generator="gen")
@GenericGenerator(name="gen", strategy="foreign", parameters=@Parameter(name="property", value="empl"))
private Long employeeId;

@Column(name="street")
private String street;

@Column(name="city")
private String city;

@Column(name="state")
private String state;

@Column(name="country")
private String country;

@OneToOne
@PrimaryKeyJoinColumn
private Employee empl;
....................
}

有人可以在执行以下查询时澄清为什么 session.createQuery("from Employee").list();

A) 像 2)、3) 和 4) 这样的连接查询被执行以从 employeedetail 中获取数据？ 为什么不使用员工对象中的employeeid 值直接从employeedetail 表中获取而无需连接？

1) Hibernate: select employee0_.employee_id as employee1_0_, employee0_.birth_date as birth_da2_0_, employee0_.cell_phone as cell_pho3_0_, employee0_.firstname as firstnam4_0_, employee0_.lastname as lastname5_0_ from employee employee0_ 

2) Hibernate: select employeede0_.employee_id as employee1_1_0_, employeede0_.city as city2_1_0_, employeede0_.country as country3_1_0_, employeede0_.state as state4_1_0_, employeede0_.street as street5_1_0_, employee1_.employee_id as employee1_0_1_, employee1_.birth_date as birth_da2_0_1_, employee1_.cell_phone as cell_pho3_0_1_, employee1_.firstname as firstnam4_0_1_, employee1_.lastname as lastname5_0_1_ from employeedetail employeede0_ left outer join employee employee1_ on employeede0_.employee_id=employee1_.employee_id where employeede0_.employee_id=? 
3) Hibernate: select employeede0_.employee_id as employee1_1_0_, employeede0_.city as city2_1_0_, employeede0_.country as country3_1_0_, employeede0_.state as state4_1_0_, employeede0_.street as street5_1_0_, employee1_.employee_id as employee1_0_1_, employee1_.birth_date as birth_da2_0_1_, employee1_.cell_phone as cell_pho3_0_1_, employee1_.firstname as firstnam4_0_1_, employee1_.lastname as lastname5_0_1_ from employeedetail employeede0_ left outer join employee employee1_ on employeede0_.employee_id=employee1_.employee_id where employeede0_.employee_id=? 
4) Hibernate: select employeede0_.employee_id as employee1_1_0_, employeede0_.city as city2_1_0_, employeede0_.country as country3_1_0_, employeede0_.state as state4_1_0_, employeede0_.street as street5_1_0_, employee1_.employee_id as employee1_0_1_, employee1_.birth_date as birth_da2_0_1_, employee1_.cell_phone as cell_pho3_0_1_, employee1_.firstname as firstnam4_0_1_, employee1_.lastname as lastname5_0_1_ from employeedetail employeede0_ left outer join employee employee1_ on employeede0_.employee_id=employee1_.employee_id where employeede0_.employee_id=? 

B) 同样，对于查询 session.createQuery("from EmployeeDetail").list();

why employee info is fetched as per below select queries instead of being proxied? 


Hibernate: select employeede0_.employee_id as employee1_1_, employeede0_.city as city2_1_, employeede0_.country as country3_1_, employeede0_.state as state4_1_, employeede0_.street as street5_1_ from employeedetail employeede0_ 

Hibernate: select employee0_.employee_id as employee1_0_0_, employee0_.birth_date as birth_da2_0_0_, employee0_.cell_phone as cell_pho3_0_0_, employee0_.firstname as firstnam4_0_0_, employee0_.lastname as lastname5_0_0_ from employee employee0_ where employee0_.employee_id=? 
Hibernate: select employee0_.employee_id as employee1_0_0_, employee0_.birth_date as birth_da2_0_0_, employee0_.cell_phone as cell_pho3_0_0_, employee0_.firstname as firstnam4_0_0_, employee0_.lastname as lastname5_0_0_ from employee employee0_ where employee0_.employee_id=? 
Hibernate: select employee0_.employee_id as employee1_0_0_, employee0_.birth_date as birth_da2_0_0_, employee0_.cell_phone as cell_pho3_0_0_, employee0_.firstname as firstnam4_0_0_, employee0_.lastname as last`enter code here`name5_0_0_ from employee employee0_ where employee0_.employee_id=?

Answer 1

A) 这有点奇怪，Hibernate 可以做得更好。然而，这是因为Employee 被热切地加载到EmployeeDetail 中，Hibernate 想要避免额外的查询来为每个EmployeeDetail 加载相应的员工。它可以做得更好，因为相应的员工已经加载了第一个查询。

如果你应用我在 B) 中的建议，这应该会消失。

B) 默认情况下，一对一的关联在 JPA 中是急切的。让它变得懒惰：

@OneToOne(fetch = FetchType.LAZY)
@PrimaryKeyJoinColumn
private Employee empl;

但是，请记住here 中描述的一对一关联的行为。如果 Hibernate 无法确定一对一关系中的代理对象是否存在或者是null，那么无论如何都会执行额外的查询来检查这一点。

为了克服这个问题，如果 empl 对每个 EmployeeDetail 都是必需的（在您的用例中，没有员工就不能存在员工详细信息），然后在关联映射中指出它：

@OneToOne(fetch = FetchType.LAZY, optional = false)
@PrimaryKeyJoinColumn
private Employee empl;

这样，Hibernate 将在加载 EmployeeDetail 实体实例而不查询 employee 表时创建一个仅包含 Employee id 的代理。

Answer 2

有人可以在执行以下查询时澄清为什么 session.createQuery("from Employee").list();

这应该会发生，因为您的 employee 表必须有 3 个条目。由于它是一对一的映射，对于从员工返回的每一行，它将触发一个查询以获取其employeedetail。所以三个查询。

B) 同样，对于查询 session.createQuery("from EmployeeDetail").list();

仅为延迟加载的实体创建代理。