@PostMapping with multiple RequestBody

I'm very new to Java Spring and have a little Problem.

I wan't to send the employee_id (from employees entity) to assignment entity in the body part (in Postman). Currently it's working, because I'm sending the employee_id as a PathVariable and I'm sending the assignment data with ResponseBody.

@PostMapping("/add/{employee_id}")
public void addEmployee(@PathVariable(value = "employee_id", required = false) Long employee_id, 
@RequestBody Assignment a) {
Employee employeeById = employeeRepository.findById(employee_id)
.orElseThrow(() -> new RuntimeException("Employee not found"));
a.setEmployee(employeeById);
assignmentRepository.addAssignment(a);
}

It is working with the url localhost:8080/add/35. 35 is the employee_id and the assignment data can be send in the Body (of Postman). What I want to do is, to send the employee_id in the body too (so the url should only be /add), but I could not get it to work.

Like that:

{
"employee_id": 35,
"title": "abc"
}

Edited:

Assignment.java

public class Assignment {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
@OneToOne(optional = false)
@JoinColumn(name = "employee_id", nullable = false)
private Employee employee;
@Size(min = 3, max = 50)
private String assignment_title;
public Assignment() {
}
public Assignment(String assignment_title) {
super();
this.assignment_title = assignment_title;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getAssignment_title() {
return assignment_title;
}
public void setAssignment_title(String assignment_title) {
this.assignment_title = assignment_title;
}
public Employee getEmployee() {
return employee;
}
public void setEmployee(Employee employee) {
this.employee = employee;
}
}

AssignmentService.java

   public void addAssignment(Assignment e) {
AssignmentRepository.save(e);
}

AssignmentRepository.java

public interface AssignmentRepository extends CrudRepository<Assignment, Integer>{
}

Employee.java

public class Employee{
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
@OneToOne(optional = false)
@JoinColumn(name = "user_id", nullable = false)
private User user;
@Size(min=3, max = 100)
private String title;
@Size(min=3, max = 50)
private String staff;
private Long degree;
public Employee() {}
public Employee( String title, String staff, Long degree) {
super();
this.title = title;
this.staff = staff;
this.degree = degree;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getStaff() {
return staff;
}
public void setStaff(String staff) {
this.staff = staff;
}
public Long getDegree() {
return degree;
}
public void setDegree(Long degree) {
this.degree = degree;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
}

EmployeeRepository.java

public interface EmployeeRepository extends CrudRepository<Employee, Integer>{
Optional<Employee> findById(Long id);
}

Thanks for your help in advance!

You need to ensure your body type Assignment has such property like

public class Assignment {
    public Long employee_id;
    // ommitted
}

And get it in your method like

@PostMapping("/add")
public void addEmployee(@PathVariable(value = "employee_id", required = false) Long employee_id, @RequestBody Assignment a) {
    Long employee_id = a.employee_id;
    Employee employeeById = employeeRepository.findById(employee_id)
        .orElseThrow(() -> new RuntimeException("Employee not found"));
    a.setEmployee(employeeById);
    assignmentRepository.addAssignment(a);
}

But such implementation does not respect HTTP standards.
Please consider that your actual implementation almost respect those standards and you should rename your API this way : POST URL/v1/employees/{employee_id}
This blog explains those standards

EDIT with provided specifications:

@PostMapping(values = {"/v1/employees", "/v1/employees/{employee_id}"})
public void addEmployee(@PathVariable(value = "employee_id", required = false) Long employee_id, @RequestBody Assignment a) {
    Long yourId = employee_id != null ? employee_id : a.employee_id;
    if (yourId == null) {
        throw new RuntimeException("No id given"); // Not clean either but implemented by OP
    } else {
        Employee employeeById = employeeRepository.findById(yourId)
            .orElseThrow(() -> new RuntimeException("Employee not found"));
        a.setEmployee(employeeById);
        assignmentRepository.addAssignment(a);
    }
}

As the pathvariable is optionnal you try to get it first from url and then from your body object.

EDIT 2 : An Assignment might be created with no Employee_id
First case : You accept that an Assignment might have no Employee.
Simply change your relation from Assignment to Employee :

    @OneToOne(optional = true)
    @JoinColumn(name = "employee_id", nullable = true)
    private Employee employee;

Second case : You want to create a new Employee first and then bind it to your assignement as an Assignment should always have an Employee

@PostMapping(values = {"/v1/employees", "/v1/employees/{employee_id}"})
public void addEmployee(@PathVariable(value = "employee_id", required = false) Long employee_id, @RequestBody Assignment a) {
    Long yourId = employee_id != null ? employee_id : a.employee_id;
    Employee employeeById = yourId != null ?
        employeeRepository.findById(yourId).orElseThrow(() -> new RuntimeException("Employee not found"))
        : new Employee("title","staff",2L);
    a.setEmployee(employeeById);
    assignmentRepository.addAssignment(a);
}