Selecting generic primary key with CriteriaQuery

Question

When migrating from Hibernate Criteria api to CriteriaQuery I ran into a generic DAO for a abstract class that has a where on a common field but does a select on their id, even if the ids are totally different per class.

The old projection looks like this

criteria.setProjection(Projections.id());

Is there any way to do this in a similar way with CriteriaQuery?

Edit: Full criteria code

DetachedCriteria detachedCriteria = DetachedCriteria.forClass(MyEntity.class);
detachedCriteria.add(Restrictions.in("accountID", accounts));
detachedCriteria.setProjection(Projections.id());

EntityManager em = ...;
Criteria criteria = detachedCriteria.getExecutableCriteria((Session) em.getDelegate());
List<Integer> list = criteria.list();

@JEY the query that hibernate runs from this code looks like this "Hibernate: select TOP(?) this_.profilePK as y0_ from Profile this_ where this_.accountID in (?)" so there really isnt much to it. — Jeppz, May 28 '18 at 08:55

score 3 · Accepted Answer · answered May 28 '18 at 11:52

3

I just managed to find it on my own.

criteriaQuery.select(
    root.get(entityRoot.getModel().getDeclaredId(int.class))
);

answered May 28 '18 at 11:52

Jeppz

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Michail Michailidis · Answer 2 · 2019-05-08T21:17:39.667

Combining answers:

https://stackoverflow.com/a/16911313

https://stackoverflow.com/a/47793003

I created this method:

public String getIdAttribute(EntityManager em, String fullClassName) {
    Class<? extends Object> clazz = null;
    try {
        clazz = Class.forName(fullClassName);
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    Metamodel m = em.getMetamodel();
    IdentifiableType<T> of = (IdentifiableType<T>) m.managedType(clazz);
    return of.getId(of.getIdType().getJavaType()).getName();
}

then I have the entity manager injected

@PersistenceContext
private EntityManager em;

I get the root entity primary key like that:

String rootFullClassName = root.getModel().getJavaType().getName();
String primaryKeyName = getIdAttribute(em, rootFullClassName);

and I get the primary keys referenced on attributes like that:

return (Specification<T>) (root, query, builder) -> {
    Set<Attribute<? super T, ?>> attributes = root.getModel().getAttributes();
    for (Attribute a: attributes) {
        if(a.isAssociation()) { 
            Path rootJoinGetName = root.join(a.getName());
            String referencedClassName = rootJoinGetName.getJavaType().getName();
            String referencedPrimaryKey = getIdAttribute(em, referencedClassName);
            //then I can use it to see if it is equal to a value (e.g 
            //filtering actors by movies with id = 1 - in 
            //this case referencedPrimaryKey is "id")
            Predicate p = rootJoinGetName.get(referencedPrimaryKey).in(1);
        }
    }
}

In this way I don't need to know the type of the primary key/referenced key in advance as it can be derived through the Entity Manager Meta Model. The above code can be used with CriteriaQuery as well as Specifications.

Selecting generic primary key with CriteriaQuery

2 Answers2