【发布时间】：2017-10-21 13:47:10
【问题描述】：
JPA 和休眠的新手，所以请耐心等待我尝试绘制完整的图片。所以我有两个具有双向关系的 Java 对象。 Employee 类是拥有类，Department 类是关系的反面。一个部门可以有很多员工，一个员工只能有一个部门。我将employee_id 指定为员工实体的主键，将department_id 指定为部门实体的主键。我还想使用 department_id 作为员工类中的外键。
员工类
@Entity
@Table(name = "Employee")
public class Employee
implements java.io.Serializable
{
private static final long serialVersionUID = 1L;
//primary key
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "Employee_ID")
private Integer id;
//foreign key
@ManyToOne(cascade=CascadeType.ALL)
@JoinColumn(name = "Department_ID")
private Department department;
@Column(name = "Employee_name")
private String name;
public Employee() { }
public Employee(Integer id)
{
this.setId(id);
}
public void setDepartment(Department department){
this.department = department;
}
public Department getDepartment(){
return department;
}
public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
public String getEmployeeName(){
return name;
}
public void setEmployeeName(String name){
this.name = name;
}
部门类
@Entity
@Table(name = "Department")
public class Department
implements java.io.Serializable
{
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "Department_ID")
private Integer id;
@Column(name = "Department_name")
private String name;
@OneToMany(mappedBy="department", cascade=CascadeType.ALL)
private Collection<Employee> employees = new ArrayList<Employee>();
public Collection<Employee> getEmployees() {return employees;}
public void setEmployees(Collection<Employee> e){ employees = e;}
public Department() { }
public Department(Integer id)
{
this.setId(id);
}
public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
public String getDepartmentName(){
return name;
}
public void setDepartmentName(String name){
this.name = name;
}
我用来生成两个对象并将它们保存在数据库中的函数。我正在测试的数据库是 H2，persistence.xml 文件最初配置为删除所有表，因此在调用以下函数之前所有表都是空的。
public static void populateTable() {
System.out.println("Populating tables");
Department d1 = new Department();
//d1.setId(1);
d1.setDepartmentName("finance");
//service.addDepartment(d1);
Employee e1 = new Employee();
e1.setEmployeeName("John");
//e1.setId(1);
//e1.setDepartment(d1);
//d1.getEmployees().add(e1);
service.addDepartment(d1);
service.addEmployee(e1);
}
最初，我尝试自己设置创建对象的主 ID，但出现“分离实体传递给持久”错误。在阅读了类似的question 之后，我了解到如果我尝试在持久化对象之前设置唯一标识符，JPA 会认为该实体已经存在于数据库中，并且会抛出“传递给持久化错误的分离实体”。所以我注释掉了 setter 函数的调用者，上面的代码运行良好。但是，如果我尝试将员工链接到部门，就像我对e1.setDepartment(d1); 所做的那样，我会收到此错误detached entity passed to persist: com.javatunes.jpa.catalog.Department。我已经阅读了很多博客，例如this one 和this post，但我仍然找不到解决这个问题的方法。还有一件事，两个持久化函数被定义为
public void addEmployee(Employee employee) {
em.persist(employee);
em.flush();
}
public void addDepartment(Department department){
em.persist(department);
em.flush();
}
在 em.persistent(employee) 处引发错误。
【问题讨论】：
标签： hibernate jpa persistence h2 ejb-3.0