使用springboot继承单个表

Question

我正在使用springboot，使用单个表进行继承，我在使用add函数时遇到问题，如何自动插入discriminator列值， ps：我不使用entityManager。 谢谢 ：） 这是3个实体和添加功能的代码

@Entity 
@Inheritance(strategy=InheritanceType.SINGLE_TABLE) 
@DiscriminatorColumn(name="Type_res", 
  discriminatorType=DiscriminatorType.STRING, length=4) 
public abstract class Reservation implements Serializable {....}

@Entity 
@DiscriminatorValue("TRAIN") 
public class Training extends Reservation{ ... }

@Entity @DiscriminatorValue("MEET") 
public class Meeting extends Reservation{ ... }

@PostMapping("/reservations/training/{spaceId}/{userId}") 
public Training addTraining(@PathVariable Long spaceId, @PathVariable(value = "userId") Long userId, @Valid @RequestBody Training training) 
{ 
    User user = userRepository.findById(userId) .orElseThrow(() -> new ResourceNotFoundException("User", "id", userId)); 
    return spaceRepository.findById(spaceId) .map(space -> { 
        training.setSpace(space); 
        training.setCourse (training.getCourse());                         
        training.setStatus(training.getStatus());             
        training.setResponsable(training.getResponsable()); 
        training.setEnd_date(training.getEnd_date()); 
        training.setStart_date(training.getStart_date()); 
        training.setUser(user); 
        return trainingRepository.save(training); }
    )
    .orElseThrow(() -> new ResourceNotFoundException("Space", "id " ,spaceId)); }