1.Here is Catagory  entity 

@NoArgsConstructor

@AllArgsConstructor

@Setter

@Getter

@Entity

@Table(name = “catagory”)

public class Catagory {

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

@Column(name = "catagory_name")
private  String catName;
@Column(name = "image")
private  String image;


@OneToMany(mappedBy = "catagory",cascade = CascadeType.ALL)

List<Item> items= new ArrayList<>() ;

}

2.Here is Item entity

@NoArgsConstructor

@AllArgsConstructor

@Getter

@Setter

@Entity

@Table(name = “items”)

public class Item {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)

private int id;

@Column(name = "cat_name")
private  String catagoryNames;


@ManyToOne
@JoinColumn(name = "cat_id",referencedColumnName = "id")
private  Catagory catagory;

}

3.rest controller

@RestControlle

r
public class CatagoryController {

@Autowired
CatagoryService catagoryService;



@PostMapping("/post")
 public void post(@RequestBody CatagoryDTO catagoryDTO) {

    catagoryService.saveByDTO(catagoryDTO);
    }

}

4. implementation

public interface CatagoryService {

void add(Catagory catagory);

void saveByDTO(CatagoryDTO catagoryDTO);

}

I did’t find this problem before. unidirectional mapping does’t gives forenkey null