How to find an object in an List of object and save by its ID - SpringBoot

Question

I have a List of Book objects that contain bookId .

I have to pass that List of Book objects and save each object in its row in database.

List looks like this:

[
    {
        "bookId": 2
    },
    {
        "bookId": 3
    },
    {
        "bookId": 2
    },
    {
        "bookId": 3
    }
]

Could someone guide me how the code should look like?

I started something like this:

public void addMultipleRents(List<RentDto> rentDtoList, long userId){
        List<Rent> rentList = Arrays.asList(modelMapper.map(rentDtoList, Rent.class));

        RentDto rentDto = new RentDto();
        
        User user = userRepository.findById(userId)
                .orElseThrow(()-> new UserNotFoundException("User with id: " + 
        rentDto.getUserId() + " is not found"));
         
        List<Rent> rent = new ArrayList<>();

        for(Rent r  : rentList){
            r.setRentStart(LocalDateTime.now());
            r.setUser(user);
            r.setBook(book);
            rent.add(r);
        }

        rentRepository.saveAll(rentList);
}

Rent.java

public class Rent {

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

    private LocalDateTime rentStart;
    private LocalDateTime rentEnd;


    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "book_id", referencedColumnName = "id")
    private Book book;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id", referencedColumnName = "id")
    private User user;

}

RentDto.java

public class RentDto {

private Long rentId;

private Long userId;

private Long bookId;

private String userFirstName;

private String userLastName;

private String bookTitle;

private LocalDateTime rentStart;

private LocalDateTime RentEnd;

}

But I am getting error for r.setBook(book); :

The given id must not be null!

Answer 1

To summarize the trail of comments:

Instead of relying on modelMapper (whatever the class is) you could iterate over the list of DTOs yourself and create a Rent entity for each of those. You then put them into a list and save that list as you're doing already.

The pseudo code for this could be something like this (simplified):

User user = userRepo.getById(userId); //get the user for all rentals

List<Rent> rentList = new ArrayList<>(rentDtos.size()); //you already know the list size you're expecting
for( RentDTO dto : rentDtos) {
  Book book = bookRepo.getById( dto.getBookId() ); //the book for the one rental
  Rent rent = new Rent(); //create a new rent
  ... //set user, book and rent time here
  rentList.add(rent);
}

rentRepository.saveAll(rentList);

Note that this (pseudo) code isn't meant to be compilable, fast or to handle all possible errors but to get you started.

One obvious improvement would be to first collect a set of book ids, load all books for those ids (eg as a Map<Long, Book> ) and then get the books to assignto the Rent entities from that map. I'll leave that as an exercise for you though.

One final advice: since you're a beginner you should first get a good grasp of the basics before getting too deep into complex frameworks such as Spring etc. Those frameworks make use of concepts like dependency injection, object-relational mapping etc. which would be too hard to understand correctly if you're still missing the basics.