Question

I would like to select only specific columns (ex. SELECT a FROM b). I have a generic DAO and what I came up with is:

public List<T> getAll(boolean idAndVersionOnly) {
    CriteriaBuilder builder = manager.getCriteriaBuilder();
    CriteriaQuery<T> criteria = builder.createQuery(entityClazz);
    Root<T> root = criteria.from(entityClazz);
    if (idAndVersionOnly) {
        criteria.select(root.get("ID").get("VERSION")); // HERE IS ERROR
    } else {
        criteria.select(root);
    }
    return manager.createQuery(criteria).getResultList();
}

And the error is:
The method select(Selection<? extends T>) in the type CriteriaQuery<T> is not applicable for the arguments (Path<Object>). How should I change that? I want to get a type T object that has only ID and VERSION fields, and all others are null.

Type T extends AbstractEntity which has those 2 fields.

entityClazz is T.class.

Answer 1

One of the JPA ways for getting only particular columns is to ask for a Tuple object.

In your case you would need to write something like this:

CriteriaQuery<Tuple> cq = builder.createTupleQuery();
// write the Root, Path elements as usual
Root<EntityClazz> root = cq.from(EntityClazz.class);
cq.multiselect(root.get(EntityClazz_.ID), root.get(EntityClazz_.VERSION));  //using metamodel
List<Tuple> tupleResult = em.createQuery(cq).getResultList();
for (Tuple t : tupleResult) {
    Long id = (Long) t.get(0);
    Long version = (Long) t.get(1);
}

Another approach is possible if you have a class representing the result, like T in your case. T doesn’t need to be an Entity class. If T has a constructor like:

public T(Long id, Long version)

then you can use T directly in your CriteriaQuery constructor:

CriteriaQuery<T> cq = builder.createQuery(T.class);
// write the Root, Path elements as usual
Root<EntityClazz> root = cq.from(EntityClazz.class);
cq.multiselect(root.get(EntityClazz_.ID), root.get(EntityClazz_.VERSION));  //using metamodel
List<T> result = em.createQuery(cq).getResultList();

See this link for further reference.