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我有一個關於JPA循環關係的問題，特別是在Eclipselink JPA實現中。對不起，如果問題有點長，但我儘可能精確。JPA/Eclipselink如何處理循環/循環關係
讓我們以部門和員工的一個簡單示例爲例，該部門具有一對多「員工」關係（並且因此是員工與部門之間的多對一「部門」關係）。現在讓我們從部門向員工添加一對一的關係「經理」（該部門的一名員工是該部門的經理）。這引入了兩個實體之間的循環關係，兩個表都將有一個引用另一個表的外鍵。
我希望能夠做到所有插入而不會導致外鍵約束違例。因此，我的想法是首先插入所有員工（不設置部門關係），然後插入部門（並設置其經理），並最終更新所有員工以設置部門。
我知道我可以使用flush()來強制插入執行的順序，但我被告知應該避免它，因此想知道是否有辦法告訴JPA/Eclipselink首先插入Department，然後Employee。
在Eclipselink中，我確實嘗試將Employee添加爲Department類的類描述符的約束依賴項，但它仍然會隨機給出錯誤。
這裏是示出這樣的代碼示例（該問題是隨機發生的）：
Department類：
package my.jpa.test;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Persistence;
/**
* Entity implementation class for Entity: Department
*
*/
@Entity
public class Department implements Serializable {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@OneToMany(fetch = FetchType.EAGER)
private List<Employee> employees;
@OneToOne
@JoinColumn(name = "manager", nullable = false)
private Employee manager;
private static final long serialVersionUID = 1L;
public Department() {
super();
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public List<Employee> getEmployees() {
return employees;
}
public void setEmployees(List<Employee> employees) {
this.employees = employees;
}
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("test-jpa");
EntityManager em = emf.createEntityManager();
Department d = new Department();
Employee manager = new Employee();
manager.setLastName("Doe");
d.setManager(manager);
Employee e1 = new Employee();
e1.setLastName("Doe");
Employee e2 = new Employee();
e2.setLastName("Smith");
em.getTransaction().begin();
em.persist(d);
manager.setDepartment(d);
e1.setDepartment(d);
e2.setDepartment(d);
em.persist(e1);
em.persist(e2);
em.persist(manager);
em.persist(d);
manager.setDepartment(d);
e1.setDepartment(d);
e2.setDepartment(d);
em.merge(manager);
em.merge(e1);
em.merge(e2);
em.getTransaction().commit();
em.clear();
Department fetchedDepartment = em.find(Department.class, d.getId());
System.err.println(fetchedDepartment.getManager().getLastName());
System.err.println(new ArrayList<Employee>(fetchedDepartment.getEmployees()));
}
public Employee getManager() {
return manager;
}
public void setManager(Employee manager) {
this.manager = manager;
}
}
Employee類：
package my.jpa.test;
import java.io.Serializable;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToOne;
/**
* Entity implementation class for Entity: Employee
*
*/
@Entity
public class Employee implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String lastName;
@ManyToOne
private Department department;
@OneToOne(mappedBy = "manager")
private Department managedDepartment;
public Employee() {
super();
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
public Department getManagedDepartment() {
return managedDepartment;
}
public void setManagedDepartment(Department managedDepartment) {
this.managedDepartment = managedDepartment;
}
@Override
public String toString() {
return "Employee " + getLastName();
}
}
的persistence.xml：
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0"
xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
<persistence-unit name="test-jpa">
<class>my.jpa.test.Department</class>
<class>my.jpa.test.Employee</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="org.h2.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:h2:mem:db1;DB_CLOSE_DELAY=-1;MVCC=TRUE" />
<property name="javax.persistence.jdbc.user" value="sa" />
<property name="javax.persistence.jdbc.password" value="" />
<property name="hibernate.hbm2ddl.auto" value="create-drop" />
<property name="eclipselink.ddl-generation" value="drop-and-create-tables" />
<property name="eclipselink.ddl-generation.output-mode" value="database" />
<property name="eclipselink.logging.level" value="FINE"/>
<property name="eclipselink.logging.parameters" value="true"/>
</properties>
</persistence-unit>
</persistence>
只Maven依賴：
<dependencies>
<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>eclipselink</artifactId>
<version>2.5.1</version>
</dependency>
<dependency>
<groupId>com.h2database</groupId>
<artifactId>h2</artifactId>
<version>1.3.172</version>
</dependency>
</dependencies>
你真的需要oneToOne是bidirectionnal？ – Gab
@Gab，可能不是，但我不認爲它會改變任何東西，如果我從Employee實體中刪除它。通過在那裏定義它，我們只增強了JPA的元模型，但由於它沒有強制執行一個外鍵約束，Eclipselink在插入順序計算時不會考慮它（據我瞭解EL）。 –
只需在關係上使用'CascadeType.PERSIST'即可。 –