为什么关联表无法使用Hibernate正确更新？

Question

I have created Employee and Department entities. These are having bidirectional many-to-one and one-to-many relation.

Using REST client, I add department and employees.

Adding Department:

{
  "name":"IT"
}

Adding Employee:

{"name": "Emp01" ,
 "address": 
    {
     "street" :"Street01" ,
     "city" :"City01" ,
     "state" :"State01" ,
     "country" :"Country01"
    },
 "department":{"id":"1"}
}

Now, I want to add a new department by associating an existing employee.

{
  "name":"Admin",
  "employees":
  [{
      "id":3
    }]
}

Now, department is added successfully but employee table is not updated properly. Except id other fields are blanked out. Ideally, hibernate should have updated employee with latest department id. Please let me know what could be missing here? Thanks.

Employee.java

package com.empcatalogue.model;

import javax.persistence.CascadeType;
import javax.persistence.Embedded;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;

    private String name;

    @Embedded
    private Address address;

    @ManyToOne(cascade = CascadeType.MERGE)
    private Department department;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }
}

Department.java

package com.empcatalogue.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;

    private String name;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "department", fetch = FetchType.EAGER)
    private List<Employee> employees = new ArrayList<>();

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public List<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }

}

EmployeeRepository.java

...
...
...    
public void addEmployee(Employee emp) {


    Session session = RepoManager.getSessionFactory().openSession();
        try {
            session.beginTransaction();

            session.save(emp);

            session.getTransaction().commit();
        } catch (Exception e) {
            session.getTransaction().rollback();
        } finally {
            session.close();
        }
    }

DepartmentRepository.java

...
...
...
    public void addDepartment(Department department) {
        Session session = RepoManager.getSessionFactory().openSession();
        try {
            session.beginTransaction();

            session.save(department);

            session.getTransaction().commit();
        } catch (Exception e) {
            session.getTransaction().rollback();
        } finally {
            session.close();
        }

    }

Answer 1

You're using javax.persistence package for your implementation, but you tagged your question as being Hibernate . Hibernate is just one of the implementations of the JPA (Java Persistence API). Even the Hibernate Web page recommends to use JPA. Unless you're maintaining a legacy code, you better use JPA API to persist and fetch your entities.

Now to your question:

Now, I want to add a new department by associating an existing employee.

To be able to persist as well as update your entities you should follow the following steps (code lines are just pseodo code):

• Create an instance of Department entity:

   Department dept = new Department(); dept.setName(...); 

• Fetch an existing Employee entity from the database within a transaction:

   Employee emp = session.get(Employee.class, <employee_id_here>); 

• Wire the entity instances together:

   dept.getEmployees().add(emp); emp.setDepartment(dept); 

• Persist the employee:

   session.save(dept); 

You should do all these in a transaction and everything should work as expected.