@Entity
@NoArgsConstructor
@Getter
@Setter
public class Product {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private Double price;
    private Integer quantity;
    @ManyToOne
    private Category category;
    @ManyToOne
    private Manufacturer manufacturer;
    public Product(String name, Double price, Integer quantity, Category category, Manufacturer manufacturer) {
        this.name = name;
        this.price = price;
        this.quantity = quantity;
        this.category = category;
        this.manufacturer = manufacturer;
}

2021-12-06 18:03:02.013 ERROR 3720 --- [nio-9090-exec-3] o.h.engine.jdbc.spi.SqlExceptionHelper   : ERROR: null value in column "id" of relation "product" violates not-null constraint
  Detail: Failing row contains (null, Dress, 333, 33, 1, 1).
2021-12-06 18:03:02.028 ERROR 3720 --- [nio-9090-exec-3] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [id" of relation "product]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement] with root cause
org.postgresql.util.PSQLException: ERROR: null value in column "id" of relation "product" violates not-null constraint
  Detail: Failing row contains (null, Dress, 333, 33, 1, 1).

@Override
    public Optional<Product> save(String name, Double price, Integer quantity, Long categoryId, Long manufacturerId) {
        Category category = this.categoryRepository.findById(categoryId).orElseThrow(() -> new CategoryNotFoundException(categoryId));
        Manufacturer manufacturer = this.manufacturerRepository.findById(manufacturerId).orElseThrow(() -> new ManufacturerNotFoundException(manufacturerId));
        this.productRepository.deleteByName(name);
        return Optional.of(this.productRepository.save(new Product(name, price, quantity, category, manufacturer)));
    }

CREATE TABLE IF NOT EXISTS public.product
    id bigint NOT NULL,
    name character varying(255) COLLATE pg_catalog."default",
    price double precision,
    quantity integer,
    category_id bigint,
    manufacturer_id bigint,
    CONSTRAINT product_pkey PRIMARY KEY (id),
    CONSTRAINT fk1mtsbur82frn64de7balymq9s FOREIGN KEY (category_id)
        REFERENCES public.category (id) MATCH SIMPLE
        ON UPDATE NO ACTION
        ON DELETE NO ACTION,
    CONSTRAINT fkq5vxx1bolpwm1sngn6krtwsjn FOREIGN KEY (manufacturer_id)
        REFERENCES public.manufacturers (id) MATCH SIMPLE
        ON UPDATE NO ACTION
        ON DELETE NO ACTION
TABLESPACE pg_default;
ALTER TABLE public.product
    OWNER to wp;

CREATE TABLE IF NOT EXISTS public.category
    id bigint NOT NULL DEFAULT nextval('category_id_seq'::regclass),
    description character varying(4000) COLLATE pg_catalog."default",
    name character varying(255) COLLATE pg_catalog."default",
    CONSTRAINT category_pkey PRIMARY KEY (id)
TABLESPACE pg_default;