Set product category over category_id

I am trying to find a way to set a category for a product over category_id but I don't know how could I use to id to set the category

My productDTO with just the basics:

public class ProductDTO {

private Integer id;





private String title;


private Category category;


private BigDecimal price;


private LocalDateTime createdAt;


private Integer category_id;

My product service

public class ProductService {

@Autowired
ProductRepository productRepository;

@Autowired
ProductMapper productMapper;


public void addProduct(ProductDTO productDTO){
    productDTO.setCreatedAt(LocalDateTime.now());

    productDTO.setCategory(productDTO.getCategory());
    Product productEntity= productMapper.convertToEntity(productDTO);

    productRepository.save(productEntity);
}}

I tried doing it with this service class but it obviously couldn't work... The error : "org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of pavicevic.koArtwebshop.entity.Category (although at least one Creator exists): no int/Int-argument constructor/factory method to deserialize from Number value (1); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException"

Edit: Here is my POST method

 @PostMapping(value = "api/products/add")
public ResponseEntity<Void> createProduct(@RequestBody ProductDTO productDTO) {
    productService.addProduct(productDTO);
    
    return new ResponseEntity<>(HttpStatus.CREATED);
}