hibernate kilka pytań nowicjuszki

@Entity
@Table(name = "clients")
public class Client implements Serializable {

//    @Temporal(TemporalType.TIMESTAMP)
//    @Column(name = "last_update", nullable = false)
//    private Date lastUpdate;
//
//    @Temporal(TemporalType.TIMESTAMP)
//    @Column(name = "created", nullable = false)
//    private Date created;
//
//    @PrePersist
//    protected void onCreate() {
//        created = lastUpdate = new Date();
//    }
//
//    @PreUpdate
//    protected void onUpdate() {
//        lastUpdate = new Date();
//    }

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "client_id", unique = true)
    private Long id;

    @Column(name = "name", nullable = false)
    private String name;

    @ManyToMany(cascade = CascadeType.ALL, mappedBy = "clients")
    private Set<Product> products = new HashSet<>();

    public Set<Product> getProducts() {
        return products;
    }

@Entity
@Table(name = "products")
public class Product implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "product_id", unique = true)
    private Long id;

    @Column(name = "name", nullable = false, unique = false)
    private String name;

    @Column(name = "price", nullable = false)
    private Integer price;

    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(
            name = "products_to_clients",
            joinColumns = {@JoinColumn(
                    name = "product_id",
                    referencedColumnName = "product_id")},
            inverseJoinColumns = {@JoinColumn(
                    name = "client_id",
                    referencedColumnName = "client_id")}
    )
    private Set<Client> clients = new HashSet<>();

    public Set<Client> getClients() {
        return clients;
    }

    //TODO: dlaczego nie zapisuje relacji ?
    @Test
    public void shouldFindAllProductsByClientNIEZAPISUJERELACJI() {
        //given
        Client client1 = helper.createRndClient();
        Client client2 = helper.createRndClient();
        client1.getProducts().add(helper.createRndProduct());
        client2.getProducts().add(helper.createRndProduct());
        client2.getProducts().add(helper.createRndProduct());
        clientDao.save(client1);
        clientDao.save(client2);

        //when
        List<Product> productsC1 = productDao.findAllProductsByClient(client1);
        List<Product> productsC2 = productDao.findAllProductsByClient(client2);

        //then
        Assert.assertEquals(1, productsC1.size());
        Assert.assertEquals(2, productsC2.size());
    }

    //a takie cos zapisuje wszystko lacznie z relacjami
    @Test
    public void shouldFindAllProductsByClient() {
        //given
        Client client1 = helper.createRndClient();
        Client client2 = helper.createRndClient();

        Product product1 = helper.createRndProduct();
        Product product2 = helper.createRndProduct();
        Product product3 = helper.createRndProduct();

        product1.getClients().add(client1);
        product2.getClients().add(client2);
        product3.getClients().add(client2);

        productDao.save(product1);
        productDao.save(product2);
        productDao.save(product3);


        //when
        List<Product> productsC1 = productDao.findAllProductsByClient(client1);
        List<Product> productsC2 = productDao.findAllProductsByClient(client2);

        //then
        Assert.assertEquals(1, productsC1.size());
        Assert.assertEquals(2, productsC2.size());
    }

    List<Client> findAllClientsByProduct(Product product);
    //vs
    List<Client> findAllClientsByProduct(Integer productId);