request spring boot ,add rating - spring

Question

i have to add rating to product by user,actually,i the user have the right to do the rating of product many times,now i will try to add rating to product once by user,if he haven't a rating to the product x,he can rate it,if he rated it in the past the value of rating will be updated:
my old request:
#Override
public RatingDto addRating(RatingDto rating) {
Optional<User> optionalUser = userRepo.findById(rating.getUser().getId());
Optional<Product> optionalProduct = productRepo.findById(rating.getProduct().getId());
rating.setUser(optionalUser.isPresent() ? optionalUser.get() : null);
rating.setProduct(optionalProduct.isPresent() ? optionalProduct.get() : null);
return ModelMapperConverter.map(ratingRepo.save(ModelMapperConverter.map(rating, Rating.class)), RatingDto.class);
}
my class rating:
public class Rating {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private double value;
#ManyToOne (cascade = CascadeType.MERGE)
#OnDelete(action = OnDeleteAction.CASCADE)
#JoinColumn(name = "userId")
private User user;
#ManyToOne (cascade = CascadeType.MERGE)
#OnDelete(action = OnDeleteAction.CASCADE)
#JoinColumn(name = "productId")
private Product product;
public Rating(double value, User user, Product product) {
this.setValue(value);
this.user = user;
this.product = product;
}
}
my class dto rating:
public class RatingDto {
private Long id;
private double value;
private User user;
private Product product;
public RatingDto(double value, User user, Product product) {
super();
this.setValue(value);
this.user = user;
this.product = product;
}
}
my table actual table in the database:
i need to have a table that contain rating of product by user and ,when he need to add a rating value,just the value will be updated