@Basic(optional = false)
    @Column(name = "fecha_alta", nullable = false, insertable = false, updatable = false, columnDefinition = "timestamp with time zone NOT NULL DEFAULT now()")
    @Temporal(value = TemporalType.TIMESTAMP)
    private Date fechaAlta;
    @Column(name = "ultima_compra")
    @Temporal(value = TemporalType.DATE)
    private Date ultimaCompra;
    @JoinColumn(name = "iva", referencedColumnName = "id", nullable = false)
    @ManyToOne(optional = false)
    private Iva iva;

public class Iva implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @Column(name = "id", nullable = false)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    @Basic(optional = false)
    @Column(name = "iva", nullable = false)
    private float iva;

    Iva findByProducto(Integer productoID) {
        CriteriaBuilder cb = getEntityManager().getCriteriaBuilder();
        CriteriaQuery<Iva> query = cb.createQuery(Iva.class);
        Root<Producto> from = query.from(Producto.class);
        query.select(from.get(Producto_.iva)).where(cb.equal(from.get(Producto_.id), productoID));
        return getEntityManager().createQuery(query).getSingleResult();
    }