使用主键联接列进行一对一映射

Question

我使用主键联接列（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;
....................
}

有人可以说明执行以下查询会话时的原因吗。createQuery（“ from Employee”）。list（）;

A）执行类似2），3）和4）的联接查询以从employeedetail中获取数据？为什么不使用雇员对象中的employeeid值直接从employeedetail表中获取数据而不进行联接？

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）有点奇怪，而且Hibernate可以做得更好。但是，这是由于这样的事实：Employee加载急切EmployeeDetail，并且Hibernate希望避免额外的查询来为每个加载相应的雇员EmployeeDetail。这样做会做得更好，因为相应的员工已经加载了第一个查询。

如果您在B）中应用我的建议，该信息应该会消失。

B）默认情况下，JPA中渴望一对一关联。使其变得懒惰：

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

但是，请记住此处描述的一对一关联的行为。如果Hibernate无法确定是否存在一对一关系的代理对象null，则无论如何将执行附加查询以检查它。

为了克服这个问题，如果empl每个参数都是强制性的EmployeeDetail（并且在您的用例中是必需的，那么没有该员工就不能存在一个员工详细信息），然后在关联映射中进行指示：

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

这样，Employee当加载EmployeeDetail实体实例而employee根本不查询表时，Hibernate将使代理仅包含id 。