@EmbeddedId
    protected SluchPK sluchPK;

@Embeddable
public class SluchPK implements Serializable {
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 36)
    @Column(name = "ID_PAK", nullable = false, length = 36)
    private String idPak;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 36)
    @Column(name = "ID_SLUCH", nullable = false, length = 36)
    private String idSluch;

        CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
        CriteriaQuery<Object> criteriaQuery = criteriaBuilder.createQuery();
        Root<Object> from = criteriaQuery.from(classname);
        CriteriaQuery<Object> select = criteriaQuery.select(from);
...
...
Expression myexp = from.get("Что тут писать???").as(someclass); // Я не знаю что тут писать - у меня всё валится от этого! EJB Transaction Exception!
mypredicate = criteriaBuilder.equal(myexp, some_var);
...
...
criteriaQuery.where(main_predicate);
TypedQuery<Object> typedQuery = em.createQuery(select);
some_ret = typedQuery.getResultList();

        CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
        CriteriaQuery<Object> criteriaQuery = criteriaBuilder.createQuery();
        Root<Object> from = criteriaQuery.from(classname);
        CriteriaQuery<Object> select = criteriaQuery.select(from);
...
...
        criteriaQuery.where(main_predicate);
        TypedQuery<Object> typedQuery = em.createQuery(select);
        some_ret = typedQuery.getResultList();

        CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
        CriteriaQuery<Object> criteriaQuery = criteriaBuilder.createQuery();
        Root<Sluch> from = criteriaQuery.from(Sluch.class);
        CriteriaQuery<Object> select = criteriaQuery.select(from);
        Expression myexp = from.get("sluchPK").get("idPak").as(Sluch.class);
        Predicate mypredicate = criteriaBuilder.equal(myexp, "Hello World!");
        criteriaQuery.where(mypredicate);
        TypedQuery<Object> typedQuery = em.createQuery(select);
        Object some_ret = typedQuery.getResultList();