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
.
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.