休眠多对多标准投影

Question

EDIT> i am at a dead end... so i can continue looking for the main reason .. Please tell me how to make a simple criteria for many to many relationships which has more than one eq restrictions, for an example, how to get the person speaking eng & german in the example shown here...

My situation is like this i have two classes person and languages, with an,m relationship.. And i am using a criteria to do the search - get all the persons which speak ex. English and German

@Entity
public class Person implements Serializable {
    private int id;
           ...........
    private Set<Languages> languages = new HashSet<Languages>();
       ...............
    @ManyToMany
    @JoinTable(name = "link_person_languages")
    public Set<Languages> getLanguages() {
       return languages;
    }
}

@Entity
public class Languages implements Serializable {
    private int id;
    private String name;
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    @Column(nullable = false, length = 40, unique = true)
    public String getName() {
        return name;
    }

Criteria

    Criteria crit = session.createCriteria(Person.class);
    crit.setCacheable(true);
    ProjectionList projList = Projections.projectionList();
    projList.add(Projections.property("languages"));
    c = enumMap.get(attr);
    if (c.isChanged()) {
       Criteria crit2 = crit.createCriteria("languages");
       Object[] o = (Object[]) c.getAnswer();
       Conjunction con = Restrictions.conjunction();
       for (int j = 0; j < o.length; j++) {
              Criterion tmp = Restrictions.eq("id", ((Languages)o[j]).getId());
              con.add(tmp);
       }
       crit2.add(con);

    }
    crit.setProjection(projList);
    retList = crit.list();

And the funny thing is, if i set it only for one language, i get the proper list of persons, but for more than one language i get none, i rechecked my base and set one person specifficaly to speak the 2 languages. But what tips mi more than anything is that the result from the projection in the Object[] on the place where the Set languages should be, there is NULL value......

please help tnx

Answer 1

What you are doing in very old JDBC style (JDBC is what very old people used to access DB) would be something like this:

SELECT * FROM PERSON WHERE LANGUAGE_ID = 1 AND LANGUAGE_ID = 2

(just example, not exactly SQL)

And, if you run this sql it will NEVER return a single line (Very sad...) because there is no in the table with LANGUAGE_ID = 1 AND LANGUAGE_ID = 2.

I don't really know the best way to solve your problem (Hibernate is not my strongest skill), but in your case (if the languages number is not so big) i would make 2 (or 3, or a loop) of selections and join then using a simple Set in code. Not the best solution... And i will happy if someone show a better way