public class SickLeave {
       
       private Integer id;
       private String number;
       private Employee employee;
       private String fromDate;
       private String toDate;
       private Doctor doctorOut;
       private Doctor doctorClosing;
       private Diagnosis diagnosis;
       private String kind;
       private String exchange;
       
       public Integer getId() {
             return id;
       }
       public void setId(Integer id) {
             this.id = id;
       }
       public String getNumber() {
             return number;
       }
       public void setNumber(String number) {
             this.number = number;
       }
       public Employee getEmployee() {
             return employee;
       }
       public void setEmployee(Employee employee) {
             this.employee = employee;
       }
       public String getFromDate() {
             return fromDate;
       }
       public void setFromDate(String fromDate) {
             this.fromDate = fromDate;
       }
       public String getToDate() {
             return toDate;
       }
       public void setToDate(String toDate) {
             this.toDate = toDate;
       }
       public Doctor getDoctorOut() {
             return doctorOut;
       }
       public void setDoctorOut(Doctor doctorOut) {
             this.doctorOut = doctorOut;
       }
       public Doctor getDoctorClosing() {
             return doctorClosing;
       }
       public void setDoctorClosing(Doctor doctorClosing) {
             this.doctorClosing = doctorClosing;
       }
       public Diagnosis getDiagnosis() {
             return diagnosis;
       }
       public void setDiagnosis(Diagnosis diagnosis) {
             this.diagnosis = diagnosis;
       }
       public String getKind() {
             return kind;
       }
       public void setKind(String kind) {
             this.kind = kind;
       }
       public String getExchange() {
             return exchange;
       }
       public void setExchange(String exchange) {
             this.exchange = exchange;
       }
       
       public boolean isNew() {
             return (this.id == null);
       }
       
}

public class Employee {
       private Integer id;
       private String lastName;
       private String firstName;
       private String middleName; 
       private String year;
       private String numberCard;
       private Work work;
       private String address;
       private String description;
       private Category category;
       
       public Integer getId() {
             return id;
       }

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

       public String getLastName() {
             return lastName;
       }

       public void setLastName(String lastName) {
             this.lastName = lastName;
       }

       public String getFirstName() {
             return firstName;
       }

       public void setFirstName(String firstName) {
             this.firstName = firstName;
       }

       public String getMiddleName() {
             return middleName;
       }

       public void setMiddleName(String middleName) {
             this.middleName = middleName;
       }

       public String getYear() {
             return year;
       }

       public void setYear(String year) {
             this.year = year;
       }

       public String getNumberCard() {
             return numberCard;
       }

       public void setNumberCard(String numberCard) {
             this.numberCard = numberCard;
       }

       public Work getWork() {
             return work;
       }

       public void setWork(Work work) {
             this.work = work;
       }

       public String getAddress() {
             return address;
       }

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

       public String getDescription() {
             return description;
       }

       public void setDescription(String description) {
             this.description = description;
       }

       public Category getCategory() {
             return category;
       }

       public void setCategory(Category category) {
             this.category = category;
       }

       public boolean isNew() {
             return (this.id == null);
       }
}

....
@Controller
@SessionAttributes("employee")
public class PatientController {
       
       private final Logger logger = LoggerFactory.getLogger(PatientController.class);
       
       @Autowired
       private EmployeeService employeeService;
       
       @Autowired
       private WorkService workService;
       
       @Autowired
       private CategoryService categoryService;
       
       @Autowired
       private SickLeaveService sickLeaveService;
       
       @Autowired
       private DoctorService doctorService;
       
       @Autowired
       private DiagnosisService diagnosisService;
....
@RequestMapping(value = "/sickLeave/add", method = RequestMethod.GET)
             public String showAddSickLeaveForm(@ModelAttribute("employee") Employee employee,BindingResult result,Model model){
                    SickLeave sickLeave = new SickLeave();
                    
                    sickLeave.setEmployee(employee);
                    
                    Doctor doctorOut = new Doctor();
                    sickLeave.setDoctorOut(doctorOut);
                    
                    Doctor doctorClosing = new Doctor();
                    sickLeave.setDoctorClosing(doctorClosing);
                    
                    Diagnosis diagnosis = new Diagnosis();
                    sickLeave.setDiagnosis(diagnosis);
                    model.addAttribute("sickLeave", sickLeave);
                    
                    model.addAttribute("employee", employee);
                    
                    List<Doctor> doctors = doctorService.findByAll();
                    model.addAttribute("doctors", doctors);
                    
                    List<Diagnosis> diagnosList = diagnosisService.findByAll();
                    model.addAttribute("diagnosis", diagnosList);
                    return "employee/sickLeaveForm";
             }
             
             @RequestMapping(value = "/sickLeave/add", method = RequestMethod.POST)
             public String addSickLeaveForm(@ModelAttribute("SickLeave") SickLeave sickLeave, @ModelAttribute("employee") Employee employee,BindingResult result, Model model,
                           final RedirectAttributes redirectAttributes) {
  //Во первых почему вот здесь он у SickLeave он пустой? на форму никакого биндинга нет, нужно скрытый делать?
//Во вторых у аттрибута employee отсустствует единственное поле id, как так?                  
                    sickLeave.setEmployee(employee);

                    logger.debug("saveOrUpdateSickLeave() : {}", sickLeave);

                    if (result.hasErrors()) {
                           return "employee/sickLeaveForm";
                    } else {

                           // Add message to flash scope
                           redirectAttributes.addFlashAttribute("css", "success");
                           if(sickLeave.isNew()){
                             redirectAttributes.addFlashAttribute("msg", "sickLeave added successfully!");
                           }else{
                             redirectAttributes.addFlashAttribute("msg", "sickLeave updated successfully!");
                           }

                           sickLeaveService.saveOrUpdate(sickLeave);

                           // POST/REDIRECT/GET
                           return "redirect:/sickLeaveList/";
                    }
             }