我对JPA中的循环关系有疑问，尤其是Eclipselink JPA实现。对不起，如果问题有点长，但我尽量保持精确。
让我们举一个部门和员工的简单例子，其中一个部门有一对多的员工＆＃34;关系（因此反向多对一＆＃34;部门＆＃34;从员工到部门的关系）。现在让我们添加一对一的关系＆＃34;经理＆＃34;从部门到员工（该部门的一名员工是该部门的经理）。这引入了两个实体之间的循环关系，两个表都有一个引用另一个表的外键。
我希望能够在不获取外键约束违规的情况下执行所有插入操作。所以，我的想法是首先插入所有员工（不设置部门关系），然后插入部门（设置其经理），最后更新所有员工以设置他们的部门。
我知道我可以使用flush()来强制插入执行的顺序但是我被告知应该避免它，因此想知道是否有办法告诉JPA / Eclipselink应该首先插入Department，员工。
在Eclipselink中，我确实尝试将Employee添加为Department类的classdescriptor的约束依赖项，但它仍然会随机出错。
这是一个代码示例，说明了这一点（问题随机发生）：
系类：
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;
}
}
员工类：
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>
Maven依赖项：
<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>
答案 0 :(得分：1)
恕我直言，这个模型你真的没有选择。
删除也可能是一团糟
否则，您可以在部门和员工之间创建一个关联表来保存isManager属性。
或者将这最后一个放在员工表中（不是很规范但很好......）
从一般观点来看，似乎不建议在关系模型中使用循环引用： In SQL, is it OK for two tables to refer to each other?
答案 1 :(得分：-1)
我认为，如果您将Employee中的部门列配置为允许null并正确设置级联，则可以解决问题。请不要使用flush