package com.example.project2.entity;

import jakarta.persistence.*;
import lombok.*;

@Entity
@AllArgsConstructor
@NoArgsConstructor
@Data
@Table(name = "LoginUser")
public class LoginUser {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int loginID;

    private String loginEmail;
    private int loginContactNumber;
    private String loginPassword;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "fk_reUserID", referencedColumnName = "reUserID")
    private RegisteredUser registeredUser;
}

package com.example.project2.entity;

import jakarta.persistence.*;
import lombok.*;

@Entity
@AllArgsConstructor
@NoArgsConstructor
@Data
@Table(name = "RegisteredUser")
public class RegisteredUser {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int reUserID;

    private String reName;
    private String reEmail;
    private String reUserName;
    private String rePassword;

    @OneToOne(mappedBy = "registeredUser")
    private LoginUser loginUser;
}

package com.example.project2.service;
import com.example.project2.dto.LoginUserDTO;
import com.example.project2.dto.RegisteredUserDTO;
import com.example.project2.entity.LoginUser;
import com.example.project2.entity.RegisteredUser;
import com.example.project2.repo.LoginUserRepo;
import com.example.project2.repo.RegisteredUserRepo;
import com.example.project2.utill.VarList;
import jakarta.transaction.Transactional;
import org.modelmapper.ModelMapper;
import org.modelmapper.TypeToken;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

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

@Service
@Transactional

public class ProjectService {

    @Autowired
    private ModelMapper modelMapper;

    @Autowired
    private RegisteredUserRepo registeredUserRepo;

    public String saveRegisteredUser(RegisteredUserDTO registeredUserDTO){
        if (registeredUserRepo.existsById(registeredUserDTO.getReUserID())){
            return VarList.RSP_DUPLICATE;
        }else {
            registeredUserRepo.save(modelMapper.map(registeredUserDTO, RegisteredUser.class));
            return VarList.RSP_SUCCESS;
        }
    }

    @Autowired
    private LoginUserRepo loginUserRepo;

    public String saveLoginUser(LoginUserDTO loginUserDTO){
        if (loginUserRepo.existsById(loginUserDTO.getLoginID())){
            return VarList.RSP_DUPLICATE;
        }else {
            loginUserRepo.save(modelMapper.map(loginUserDTO, LoginUser.class));
            return VarList.RSP_SUCCESS;
        }
    }
    
}

package com.example.project2.controller;

import com.example.project2.dto.*;
import com.example.project2.repo.LoginUserRepo;
import com.example.project2.repo.RegisteredUserRepo;
import com.example.project2.service.ProjectService;
import com.example.project2.utill.VarList;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("api/v1/project")
public class Controller {

    @Autowired
    private ProjectService projectService;

    @Autowired
    private ResponseDTO responseDTO;

    @PostMapping(value = "/saveEmployee")
    public ResponseEntity saveEmployee(@RequestBody EmployeeDTO employeeDTO){
        try {
            String res= projectService.saveEmployee(employeeDTO);
            if (res.equals("00")){
                responseDTO.setCode(VarList.RSP_SUCCESS);
                responseDTO.setMassage("Success");
                responseDTO.setContemt(employeeDTO);
                return new ResponseEntity(responseDTO, HttpStatus.ACCEPTED);

            }else if(res.equals("06")){
                responseDTO.setCode(VarList.RSP_DUPLICATE);
                responseDTO.setMassage("Employee registered");
                responseDTO.setContemt(employeeDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);
            }else {
                responseDTO.setCode(VarList.RSP_FAIL);
                responseDTO.setMassage("Error");
                responseDTO.setContemt(null);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);
            }
        }catch (Exception ex){
            responseDTO.setCode(VarList.RSP_ERROR);
            responseDTO.setMassage("Success");
            responseDTO.setContemt(null);
            return new ResponseEntity(responseDTO, HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }
    
    @Autowired
    private AdminRepo adminRepo;

    @PostMapping(value = "/saveAdmin")
    public ResponseEntity saveAdmin(@RequestBody AdminDTO adminDTO){
        try {
            String res = projectService.saveAdmin(adminDTO);
            if(res.equals("00")){
                responseDTO.setCode(VarList.RSP_SUCCESS);
                responseDTO.setMassage("succes");
                responseDTO.setContemt(adminDTO);
                return new ResponseEntity(responseDTO, HttpStatus.ACCEPTED);

            } else if (res.equals("06")) {
                responseDTO.setCode(VarList.RSP_DUPLICATE);
                responseDTO.setMassage("Employee registerd");
                responseDTO.setContemt(adminDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);

            }else {
                responseDTO.setCode(VarList.RSP_FAIL);
                responseDTO.setMassage("Error");
                responseDTO.setContemt(adminDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);
            }
        }catch (Exception ex){
            responseDTO.setCode(VarList.RSP_ERROR);
            responseDTO.setMassage("Success");
            responseDTO.setContemt(adminDTO);
            return new ResponseEntity(responseDTO, HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }
   

    @Autowired
    private RegisteredUserRepo registeredUserRepo;

    @PostMapping(value = "/saveRegisteredUser")
    public ResponseEntity saveRegisteredUser(@RequestBody RegisteredUserDTO registeredUserDTO){
        try {
            String res = projectService.saveRegisteredUser(registeredUserDTO);
            if(res.equals("00")){
                responseDTO.setCode(VarList.RSP_SUCCESS);
                responseDTO.setMassage("succes");
                responseDTO.setContemt(registeredUserDTO);
                return new ResponseEntity(responseDTO, HttpStatus.ACCEPTED);

            } else if (res.equals("06")) {
                responseDTO.setCode(VarList.RSP_DUPLICATE);
                responseDTO.setMassage("RegisteredUser registerd");
                responseDTO.setContemt(registeredUserDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);

            }else {
                responseDTO.setCode(VarList.RSP_FAIL);
                responseDTO.setMassage("Error");
                responseDTO.setContemt(registeredUserDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);
            }
        }catch (Exception ex){
            responseDTO.setCode(VarList.RSP_ERROR);
            responseDTO.setMassage("Success");
            responseDTO.setContemt(registeredUserDTO);
            return new ResponseEntity(responseDTO, HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }
  
    @Autowired
    private LoginUserRepo loginUserRepo;

    @PostMapping(value = "/saveLoginUser")
    public ResponseEntity saveLoginUser(@RequestBody LoginUserDTO loginUserDTO){
        try {
            String res = projectService.saveLoginUser(loginUserDTO);
            if(res.equals("00")){
                responseDTO.setCode(VarList.RSP_SUCCESS);
                responseDTO.setMassage("succes");
                responseDTO.setContemt(loginUserDTO);
                return new ResponseEntity(responseDTO, HttpStatus.ACCEPTED);

            } else if (res.equals("06")) {
                responseDTO.setCode(VarList.RSP_DUPLICATE);
                responseDTO.setMassage("LoginUser registerd");
                responseDTO.setContemt(loginUserDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);

            }else {
                responseDTO.setCode(VarList.RSP_FAIL);
                responseDTO.setMassage("Error");
                responseDTO.setContemt(loginUserDTO);
                return new ResponseEntity(responseDTO, HttpStatus.BAD_REQUEST);
            }
        }catch (Exception ex){
            responseDTO.setCode(VarList.RSP_ERROR);
            responseDTO.setMassage("Success");
            responseDTO.setContemt(loginUserDTO);
            return new ResponseEntity(responseDTO, HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }
}

I make two tables named, LoginUser and RegisteredUser. then connected two tables with a foreign key. then I created save methods for above class. Then I called saveLOginUser method in postman using an Api. Then I entered a dataset for the two tables, but data is only saved in the LoginUser table and not in the RegisteredUser table, also the primary key is null. Why data is not saved in the RegisterUser table.