了解注释和JPA（hibernate）

Question

有一个结构。我想以这种方式链接这三个实体：公司应包含ID，公司名称和部门列表，每个部门都有一个工人列表，ID和部门名称。每个工人都有名字，身份证。

+Company
-int companyId
-String companyName
-Set<Department> listOfDepartments = new HashSet<Department>();

+Department
-int departmentId
-String departmentName
-Set<Worker> listOfWorkers = new HashSet<Worker>();

+Worker
-int workerId
-String workerName

我试图与一对多和多对一建立联系，但未成功。

公司

@Entity
@Table(name="Company")
public class Company {

@Id @GeneratedValue(strategy = GenerationType.AUTO)
private int idCompany;
private String companyName;

@OneToMany(mappedBy = "company")
private Set<Department> listOfDepartments = new HashSet<Department>();

部

@Entity
public class Department {

@Id @GeneratedValue(strategy = GenerationType.AUTO)
private int idDepartment;
private String departmentName;

@ManyToOne
@JoinColumn(name="idCompany")
private Company company;

@OneToMany(mappedBy = "department")
private Set<Worker> listOfWorkers = new HashSet<Worker>();

工人

@Entity
public class Worker {

@Id @GeneratedValue(strategy = GenerationType.AUTO)
private int idWorker;
private String workerName;

@ManyToOne
@JoinColumn(name="idDepartment")

我从开始：

Session session = sessionFactory.openSession();
        session.beginTransaction();
        Worker worker1 = new Worker("WorkerName1");
        Worker worker2 = new Worker("WorkerName2");
        Worker worker3 = new Worker("WorkerName3");
        Worker worker4 = new Worker("WorkerName4");
        Department department = new Department("Technical");
        department.getListOfWorkers().add(worker1);
        department.getListOfWorkers().add(worker2);
        department.getListOfWorkers().add(worker3);
        department.getListOfWorkers().add(worker4);
        company = new Company("MyCompanyName");
        company.getListOfDepartments().add(department);
        session.save(company);
        session.getTransaction().commit();
        session.close();

它填充公司，但不填充其他表，也没有创建任何联接（映射）错误：

Hibernate: alter table Department drop foreign key FK_l7sg67atqhnn0soqynpvxrtpk
Hibernate: alter table Worker drop foreign key FK_s53hyohtyjy93srd2wkksairk
Hibernate: drop table if exists Company
Hibernate: drop table if exists Department
Hibernate: drop table if exists Worker
Hibernate: create table Company (idCompany integer not null auto_increment, companyName varchar(255), primary key (idCompany))
Hibernate: create table Department (idDepartment integer not null auto_increment, departmentName varchar(255), idCompany integer, primary key (idDepartment))
Hibernate: create table Worker (idWorker integer not null auto_increment, workerName varchar(255), idDepartment integer, primary key (idWorker))
Hibernate: alter table Department add index FK_l7sg67atqhnn0soqynpvxrtpk (idCompany), add constraint FK_l7sg67atqhnn0soqynpvxrtpk foreign key (idCompany) references Company (idCompany)
Hibernate: alter table Worker add index FK_s53hyohtyjy93srd2wkksairk (idDepartment), add constraint FK_s53hyohtyjy93srd2wkksairk foreign key (idDepartment) references Department (idDepartment)
ноя 11, 2013 3:10:31 AM org.hibernate.tool.hbm2ddl.SchemaExport execute
INFO: HHH000230: Schema export complete
Hibernate: insert into Company (companyName) values (?)

Answer 1

除了Glenn Lane的答案中提到的级联，您还需要了解双向关联是如何工作的。

它们有一个所有者方面和一个相反方面。JPA只关心所有者方面，以决定实体之间存在哪个关联。该业主方是一个 不 具备的mappedBy属性。

您的代码为公司增加了部门，为部门增加了工人，但是仅初始化了相反的方面。您忘记了初始化所有者方：

worker1.setDepartment(department);
worker2.setDepartment(department);
...
department.setCompany(company);