Hibernate’s “Object References an Unsaved Transient Instance” Error

Question

I recently startet Programming a Back-End in Spring Boot. When implementing the Entitys and their Relationships I soon faced Hibernate’s “Object References an Unsaved Transient Instance” Error. I searched for answers in this forum and tried to fix my Cascade Annotation with this Tutorial: https://www.baeldung.com/hibernate-unsaved-transient-instance-error (which is addressing the specific Error). But when I start my Project and fill in Hard Code Data with the init() method I always Face the Error again:

org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : *.structure.Cart.customer -> *.structure.Customer; nested exception is java.lang.IllegalStateException: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : *.structure.Cart.customer -> *.structure.Customer

@Entity
@Table(name="arti")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Article {
    
    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long articleId;
    private String name;
    private String manufactor;
    private float price;

    @ManyToMany
    @Cascade({CascadeType.SAVE_UPDATE, CascadeType.MERGE, CascadeType.PERSIST})
    @JoinColumn(name = "cart_id")
    private Set<Cart> articleCart= new HashSet<>();

    @ManyToMany
    @Cascade({CascadeType.SAVE_UPDATE, CascadeType.MERGE, CascadeType.PERSIST})
    @JoinColumn(name = "order_id")
    private Set<Cart> articleOrder= new HashSet<>();
}

@Entity
@Table(name="cust")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Customer {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long customerId;
    @Column(name = "name")
    private String name;
    @Column(name = "address")
    private String address;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "cart_id", referencedColumnName = "id")
    private Cart cart;

    @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<Order> orders = new HashSet<>();
}

@Entity
@Table(name="cart")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Cart {
    
    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long cartId;

    @OneToOne(mappedBy = "cart")
    private Customer customer;

    @ManyToMany
    @JoinColumn(name = "article_id")
    private Set<Article> articles= new HashSet<>();
}

@Entity
@Table(name="orde")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Order {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long orderId;

    @ManyToOne
    @Cascade(CascadeType.SAVE_UPDATE)
    @JoinColumn(name = "customer_id")
    private Customer customer;

    @ManyToMany
    @JoinColumn(name = "article_id")
    private Set<Article> articles= new HashSet<>();
}

@SpringBootApplication
public class GqlApplication {

    @Autowired
    private ArticleRepository articleRepository;
    @Autowired
    private CartRepository cartRepository;
    @Autowired
    private CustomerRepository customerRepository;
    @Autowired
    private OrderRepository orderRepository;

    public static void main(String[] args) {
        SpringApplication.run(GqlApplication.class, args);
    }

    @PostConstruct
    public void init() {
        Article dress = Article.builder()
                .name("red dress")
                .manufactor("Prada")
                .price(20)
                .build();

        Article hat = Article.builder()
                .name("brown hat")
                .manufactor("Gucci")
                .price(280)
                .build();

        Set<Article> articles = new HashSet<>();
        articles.add(dress);
        articles.add(hat);

        Customer tom = Customer.builder()
                .name("tom")
                .address("Am Baum 3")
                .build();

        Cart cart = Cart.builder()
                .customer(tom)
                .articles(articles)
                .build();

        Order order = Order.builder()
                .customer(tom)
                .articles(articles)
                .build();

        Set<Order> orders = new HashSet<>();
        orders.add(order);

        tom.setCart(cart);
        tom.setOrders(orders);

        articleRepository.save(dress);
        articleRepository.save(hat);
        cartRepository.save(cart);
        customerRepository.save(tom);
        orderRepository.save(order);

    }

}