@Entity
@DynamicUpdate
@Table(name = "tank")
public class Tank {

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

    @OneToOne(cascade=CascadeType.ALL)
    @JoinColumn(name = "level_sensor_id")
    private LevelSensor levelSensor;

    public LevelSensor getLevelSensor() {
        return levelSensor;
    }

    public void setLevelSensor(LevelSensor levelSensor) {
        this.levelSensor = levelSensor;
    }
}

@Entity
@DynamicUpdate
@Table(name = "level_sensor")
public class LevelSensor {

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

    @OneToOne(mappedBy="levelSensor", cascade=CascadeType.PERSIST)
    private Tank tank;
}

@RequestMapping(value="/levelSensor/", method = RequestMethod.POST, produces = "application/json")
    @ResponseBody
    public ResponseEntity<Object> addLevelSensor(
            @RequestBody LevelSensorDTO levelSensorDTO,
            @RequestHeader("Accept") String acceptHeader, HttpSession session) {

        LevelSensor newLevelSensor= new LevelSensor();

        Tank tank = tankService.getTankById(levelSensorDTO.getTankId());

        newLevelSensor.setTank(tank);

        newLevelSensor = levelSensorService.saveLevelSensor(newLevelSensor);

        log.info("The level sensor : "+ levelSensorDTO.toString() +" was successfully added");

        return new ResponseEntity<Object>(null ,HttpStatus.OK);
    }