使用主键连接列进行一对一映射 [英] one to one mapping using primary key join column
问题描述
我使用主键连接列（employee_id）映射（双向）员工和员工详细信息类
@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日期birthDate;
@Column（name =cell_phone）
私人字符串手机;
@OneToOne（mappedBy =empl，cascade = CascadeType.ALL）
private EmployeeDetail employeeDetail;
$ b @Table（name =employeedetail）
public class EmployeeDetail {
@Id
@Column（name =employee_id，unique = true，nullable = false）
@GeneratedValue（generator =gen）
@GenericGenerator（name =gen，strategy =foreign，parameters = @参数（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
私人雇员empl;
....................
}
有人可以澄清为什么执行以下查询时session.createQuery（from Employee）。list（）; A）像2），3）和4）这样的连接查询被执行从employeedetail中获取数据？
为什么不直接从employeedetail表中使用雇员对象中的employeeid值进行获取而没有连接？
1）Hibernate：选择employee0_.employee_id为employee1_0_，employee0_.birth_date为birth_da2_0_，employee0_.cell_phone为cell_pho3_0_，employee0_.firstname作为firstnam4_0_，employee0_.lastname作为lastname5_0_从员工employee0_
$ b 2）Hibernate：选择employeede0_.employee_id作为employee1_1_0_，employeede0_city作为city2_1_0_，employeede0_.country作为country3_1_0_，employeede0_.state作为state4_1_0_，employeede0_。 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：选择employeede0_.employee_id为employee1_1_0_，employeede0_city为city2_1_0_，employeede0_.country为country3_1_0_，employeede0_.state为state4_1_0_，employeede0_.street为street5_1_0_，employee1_.employee_id为employee1_0_1_，employee1_.birth_date为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：选择employeede0_.employee_id作为employee1_1_0_，employeede0_city作为city2_1_0_，employeede__ country作为country3_1_0_，employeede0_.state作为state4_1_0_，employeede0_.street作为street5_1_0_，employee1_.employee_id作为employee1_0_1_，employee1_.birth_date作为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，查询session.createQuery（from EmployeeDetail）。
为什么员工信息是按照以下方式提取select queries而不是代理？
Hibernate：选择employeede0_.employee_id为employee1_1_，employeede0_city为city2_1_，employeede0_.country为country3_1_，employeede0_.state为state4_1_，employeede0_.street为street5_1_ from employeedetail employeede0_
Hibernate：选择employee0_.employee_id为employee1_0_0_，employee0_.birth_date为birth_da2_0_0_，employee0_.cell_phone为cell_pho3_0_0_，employee0_.firstname为firstnam4_0_0_，employee0_.lastname为lastname5_0_0_，来自employee employee0_，其中employee0_.employee_id =？
Hibernate：选择employee0_.employee_id作为employee1_0_0_，employee0_.birth_date作为birth_da2_0_0_，employee0_.cell_phone作为cell_pho3_0_0_，employee0_.firstname作为firstnam4_0_0_，employee0_.lastname作为lastname5_0_0_来自员工employee0_，其中employee0_.employee_id =？
Hibernate：选择employee0_.employee_id作为employee1_0_0_，employee0_.birth_date作为birth_da2_0_0_，employee0_.cell_phone作为cell_pho3_0_0_，employee0_.firstname作为firstnam4_0_0_，employee0_.lastname作为last`enter code here`name5_0_0_ from employee employee0_ where employee0_.employee_id = ？
如果您在B中应用我的建议，这应该会消失。
默认情况下，JPA中的一对一关联是热切的。让它懒：
@OneToOne（fetch = FetchType.LAZY）
@PrimaryKeyJoinColumn
private Employee empl的;
但是请记住此处。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它。
为了解决这个问题，如果 EmployeeDetail 必须是 empl 在您的用例中，如果没有员工，员工详细信息不能存在），然后在关联映射中指明它：
@OneToOne（ fetch = FetchType.LAZY，可选= false）
@PrimaryKeyJoinColumn
私人雇员empl;
通过这种方式，Hibernate将创建一个仅包含 Employee EmployeeDetail 实体实例而不查询 employee 表时，c> id。
I have employee and employee detail classes mapped (bi directional) using primary key join column (employee_id)
@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;
....................
}
Can someone please clarify why when executing the following query session.createQuery("from Employee").list();
A) the join queries like 2),3) and 4) get executed to fetch data from employeedetail? why not directly fetch from employeedetail table using the employeeid value in employee object without a join?
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) also, for the query 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=?
A) It is a little bit weird, and Hibernate could do it better. However, this is due to the fact that Employee is loaded eagerly in EmployeeDetail, and Hibernate wants to avoid additional queries to load the corresponding employee for each EmployeeDetail. It could do it better because the corresponding employees are already loaded with the first query.
This should disappear if you apply my advice in B).
B) By default, one-to-one associations are eager in JPA. To make it lazy:
@OneToOne(fetch = FetchType.LAZY)
@PrimaryKeyJoinColumn
private Employee empl;
However, keep in mind the behavior of one-to-one associations described here. If Hibernate cannot determine whether the proxied object in one-to-one relation exists or is null, then additional query will be executed anyway to check this.
To overcome this, if empl is mandatory for each EmployeeDetail (and it is in your use case, an employee detail cannot exist without the employee), then indicate it in the association mapping:
@OneToOne(fetch = FetchType.LAZY, optional = false)
@PrimaryKeyJoinColumn
private Employee empl;
This way Hibernate will make a proxy containing only Employee id when loading EmployeeDetail entity instances without querying employee table at all.
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