更新父对象时删除特定子对象的Spring Boot JPA技术问题

各位大佬，我现在基于Spring Boot和Spring JpaRepository做开发，部门实体Department关联了员工列表Employee，创建部门的功能没问题，但更新部门的时候遇到两个棘手的问题，想请教下大家：

实体类定义 #

Department实体 #

@Entity 
@Table(name="department") 
public class Department{ 
    @Id 
    @Column(name="dept_id") 
    @GeneratedValue(strategy=GenerationType.AUTO) 
    public Integer id; 
    @Column(name="dept_name") 
    public String name; 
    @Column(name="dept_code") 
    public String code; 
    @OneToMany(cascade = {CascadeType.REMOVE,CascadeType.REFRESH},orphanRemoval = true, fetch = FetchType.LAZY,mappedBy="department") 
    public List<Employee> employees; 
}

Employee实体 #

@Entity 
@Table(name="employee") 
public class Employee { 
    @Id 
    @Column(name="emp_id") 
    @GeneratedValue(strategy=GenerationType.AUTO) 
    public Integer id; 
    @Column(name="emp_name") 
    public String name; 
    @ManyToOne 
    @JoinColumn(name = "dept_id") 
    public Department department; 
    @Transient 
    public String operation; 
}

服务层代码 #

@Transactional 
@Service 
public class DepartmentService { 
    public Department createDepartment(Department department) { 
        Department dept = departmentRepository.save(department); 
        for (Employee emp : department.getEmployees()) { 
            emp.setDepartment(department); 
            employeeRepository.save(emp); 
        } 
        return dept; 
    } 

    public Department updateDepartment(Department department) { 
        Department dept = departmentRepository.save(department); 
        if (!dept.getEmployees().isEmpty()) { 
            for (Employee emp : department.getEmployees()) { 
                if (emp.getOperation().equalsIgnoreCase("delete")) 
                    employeeRepository.deleteById(emp.getId()); 
                else 
                    employeeRepository.save(emp); 
            } 
        } 
        return dept; 
    } 

    public Department getDepartment(int id) { 
        return departmentRepository.getOne(id); 
    } 
}

Controller代码 #

@RestController 
public class DepartmentController { 
    @Autowired 
    DepartmentService departmentService; 
    private ModelMapper modelMapper = new ModelMapper(); 

    @RequestMapping(value = "/dept", method = RequestMethod.PUT, produces = MediaType.APPLICATION_JSON_VALUE) 
    @ResponseBody 
    public ResponseEntity<DepartmentBean> updateDepartment(@RequestBody DepartmentBean deptBean) { 
        Department dept = modelMapper.map(deptBean, Department.class); 
        Department persistedDept = departmentService.updateDepartment(dept); 
        Department d = departmentService.getDepartment(persistedDept.getId()); 
        DepartmentBean userDTO = modelMapper.map(d, DepartmentBean.class); 
        return new ResponseEntity<DepartmentBean>(userDTO, HttpStatus.OK); 
    } 

    @RequestMapping(value = "/dept/{id}", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE) 
    @ResponseBody 
    public ResponseEntity<DepartmentBean> getDepartment(@PathVariable int id) { 
        Department dept = departmentService.getDepartment(id); 
        DepartmentBean userDTO = modelMapper.map(dept, DepartmentBean.class); 
        return new ResponseEntity<DepartmentBean>(userDTO, HttpStatus.OK); 
    } 
}

更新接口使用的JSON示例 #

{ 
    "id": 5, 
    "name": "DEPT061", 
    "code": "CODE061", 
    "employees": [ ] 
}

遇到的具体问题 #

1. 服务层标注了@Transactional，查询接口返回的是Department代理对象，在服务层内部没法获取员工列表，但用ModelMapper转换成DepartmentBean的时候却能拿到——为啥事务外能从代理对象获取员工列表？
2. 更新方法返回的代理对象在服务层内部没法获取员工列表，更新后Controller调用查询方法也同样拿不到员工列表。

内容的提问来源于stack exchange，提问作者shivam