Confusing situation when registering the product

Question

i have problem with my e-commerce app. I'm confused when saving the product entity. I have a Product,Brand and Category Model like this

@Getter
@Setter
@Entity
public class Product extends BaseExtendedModel {

    private String name;
    private BigDecimal price;
    private UUID barcode;
    private String image;
    @ManyToOne
    private Brand brand;
    @ManyToOne
    private Category category;
    private int stock;

}

@Getter
@Setter
@Entity
public class Brand extends BaseModel {
    private String name;
}

@Entity
@Getter
@Setter
public class Category extends BaseModel {

    @OneToOne
    private Category parent;

    private String name;
}

I used CreateProductRequestDTO to save the data

public record CreateProductRequestDTO(String name, BigDecimal price, String image, Long brandId, Long categoryId, int stock) {
}

I got the necessary DTO information from the user to register product. And to save this information as product I need to convert DTO to product. And I'm doing this convert in the ProductConverterImpl below.

@Component
public class ProductConverterImpl implements ProductConverter{
    @Override
    public Product toProduct(CreateProductRequestDTO createProductRequestDTO) {
       Product product = new Product();
       product.setName(createProductRequestDTO.name());
       product.setPrice(createProductRequestDTO.price());
       product.setImage(createProductRequestDTO.image());
        Brand brand = new Brand();
        brand.setId(createProductRequestDTO.brandId());
        Category category = new Category();
        category.setId(createProductRequestDTO.categoryId());

        product.setCategory(category);
        product.setStock(createProductRequestDTO.stock());
        product.setBrand(brand);
        product.setCreatedBy("Yunus Emre");
       return product;
    }
}

But I am confused about this. Because in my product model i have brand,category model as property marked @ManyToOne. I don't have Brand and Category object that I can set during the convert process. I am only getting brandId and categoryId from user. How can I set this Product object with the brand and category fields set according to the id information I received from the user? I need help.